RE: Does MLS (Layer 3 switching) require VLANs? [7:63147]
comments in-line: -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Kelly Cobean Sent: Tuesday, February 18, 2003 7:54 PM To: [EMAIL PROTECTED] Subject: RE: Does MLS (Layer 3 switching) require VLANs? [7:63147] Priscilla, Ok, you caught me not telling the whole truth. There is a second VLAN on the switch, but my point was that the MLS cache is full of entries for one host talking to another host off of the same VLAN interface but on a secondary subnet, indicating that L3 switching (routing) took place for that data-flow...So now I guess there are two hands clapping ;-) You sure do keep us all on our toes!!! Thanks! - that's because packet switching between subnets using secondaries are process-switched. regards, /vicky -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] Sent: Tuesday, February 18, 2003 1:08 AM To: [EMAIL PROTECTED] Subject: RE: Does MLS (Layer 3 switching) require VLANs? [7:63147] I'm loath to continue this discussion, but I do have a question for Kelly. Why do you have a VLAN at all in your example?? Isn't a single VLAN sort of like one hand clapping? Seriously, what role is it playing in your network? Of course you don't have to have VLANs to do routing/L3 switching, as you probabaly know. But maybe there's some weird configuration gotcha, specific to the 6509? Just curious. Thanks. Larry said the majority of the Cisco campus is networked with L3 switches and not using vlans. That says a lot right there! Priscilla Kelly Cobean wrote: All, I'd like to add to this something that I haven't seen in other posts yet, and that is a quick look at layer2 function. I have a Catalyst 6509 with an MSFC on it. There is only *ONE* VLAN configured on the MSFC, however, that VLAN has several secondary addresses assigned to it (I know, not a great solution, but let's not go there). If I do a show mls entry on my switch, it is full of entries for hosts talking to hosts on the same VLAN. My point? When a host wants to talk to a host on another subnet (VLAN or not), it ANDs the address with it's own mask, determines that the host is in fact on a different subnet, then arps (if necessary) for it's default gateway (the MSFC) and sends the packet on it's way. The 6509/MSFC receive the packet and begin the MLS cache setup process (candidate packet, timeout, etc). All this is still done inspite of the fact that the MSFC only has a single VLAN. -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Stephen Hoover Sent: Monday, February 17, 2003 8:33 PM To: [EMAIL PROTECTED] Subject: Re: Does MLS (Layer 3 switching) require VLANs? [7:63147] - actually it is by doing secondaries, but i would highly recommend doing vlans if possible. keep it clean and simple. one may also configure the physical interfaces as L3 interfaces - just as one might do on a router with several ethernet ports. Oo ok, now THAT statement leads me to believe the L3 switching IS possible without VLANs. -Stephen Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=63371t=63147 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: Does MLS (Layer 3 switching) require VLANs? [7:63147]
comments in-line: -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Howard C. Berkowitz Sent: Tuesday, February 18, 2003 6:42 AM To: [EMAIL PROTECTED] Subject: Re: Does MLS (Layer 3 switching) require VLANs? [7:63147] At 5:30 AM + 2/18/03, Ken Diliberto wrote: The nit I'm picking is inline... (I'm feeling like chipping in tonight) The Long and Winding Road 02/17/03 06:13PM [snip] if I have a 75xx router with 300 ethernet ports, and I bridge all those ports, do I have an L3 switch, or a router? [KD] You have a router performing L2 operations (forwarding, switching, bridging -- whatever). Would a cheap Linksys switch be faster? What makes a L3 switch in my mind is where the forwarding happens. If the L3 CPU (new way to look at it?) has to handle every packet, that's a router. If the first L3 packet is handled by the CPU which then programs ASICs to handle the rest of the flow without bothering the CPU, that's an L3 switch. Is there a difference from a packet/network perspective? No. The L2 headers and L3 headers are all properly updated in both cases (at least we *hope* they are) and traffic is delivered most of the time. (If it was delivered all the time, networks wouldn't need us to fix them) :-) Does that make a 7500 with VIPs a L3 switch? A 12000 with distributed forwarding processors? -- it dependscall it (d)cef switching router if you want but i have to kinda agree with ken's comments. in my opinion the major difference between a tradition router and a l3 switch is the way packet switching takes place. in a tradition router the packet switching are done in software (microprocessor based), whereas in l3 switch it is done by asic in hw and mls is used to increase routing performance by doing packet switching and rewrites in hw (asics). that's all. regards, /vicky Substituting router for L3 switch is a good idea, but go farther than that. You can think of a high-performance router as a small hidden network, containing one or more (think high availability) path determination routing processors/hosts that download FIB information to multiple forwarding processors/hosts. One public and vendor-independent discussion of this architecture continues in the IETF FORCES Working Group (go to www.ietf.org and navigate to Working Groups). What does this mean to us? Not much other than for capacity planning. IMHO, an L3 switch has a longer life than a router. Not really, as you say in your next paragraph. I could go off into the ozone and say all high-speed routers are L3 switches. Indeed, ASICs aren't a necessity. I've worked on research router designs that used RISC processors in each forwarding and path determination engine, which gave lots of power but much more flexibility than ASICs. Admittedly, at least one of these was a specifically designed processor, but it definitely was software loadable and ran a real time OS. ASIC gets blurry anyway, when you start getting into the pure hard-etched IC, field-programmable gate arrays, electrically alterable field-programmable gate arrays, microcode sequencers, etc. When I design networks, I don't think L3 switch. I think about routers interconnecting L2 segments. I even draw them that way most of the time. :-) My advice to those having problems with this subject: Replace every occurrence of layer 3 switch with router. [/KD] Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=63372t=63147 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: Does MLS (Layer 3 switching) require VLANs? [7:63147]
At 6:19 PM + 2/19/03, Vicky Rode wrote: comments in-line: -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Kelly Cobean Sent: Tuesday, February 18, 2003 7:54 PM To: [EMAIL PROTECTED] Subject: RE: Does MLS (Layer 3 switching) require VLANs? [7:63147] Priscilla, Ok, you caught me not telling the whole truth. There is a second VLAN on the switch, but my point was that the MLS cache is full of entries for one host talking to another host off of the same VLAN interface but on a secondary subnet, indicating that L3 switching (routing) took place for that data-flow...So now I guess there are two hands clapping ;-) You sure do keep us all on our toes!!! Thanks! - that's because packet switching between subnets using secondaries are process-switched. On general IOS -- can't speak to the switch implementations -- you can code ip route-cache same-interface ipx route-cache same-interface and get fast switching for secondaries. Don't know if there is a way for CEF to figure this out. Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=63385t=63147 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: Does MLS (Layer 3 switching) require VLANs? [7:63147]
At 6:51 PM + 2/19/03, Vicky Rode wrote: comments in-line: -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Howard C. Berkowitz Sent: Tuesday, February 18, 2003 6:42 AM To: [EMAIL PROTECTED] Subject: Re: Does MLS (Layer 3 switching) require VLANs? [7:63147] At 5:30 AM + 2/18/03, Ken Diliberto wrote: The nit I'm picking is inline... (I'm feeling like chipping in tonight) The Long and Winding Road 02/17/03 06:13PM [snip] if I have a 75xx router with 300 ethernet ports, and I bridge all those ports, do I have an L3 switch, or a router? [KD] You have a router performing L2 operations (forwarding, switching, bridging -- whatever). Would a cheap Linksys switch be faster? What makes a L3 switch in my mind is where the forwarding happens. If the L3 CPU (new way to look at it?) has to handle every packet, that's a router. If the first L3 packet is handled by the CPU which then programs ASICs to handle the rest of the flow without bothering the CPU, that's an L3 switch. Is there a difference from a packet/network perspective? No. The L2 headers and L3 headers are all properly updated in both cases (at least we *hope* they are) and traffic is delivered most of the time. (If it was delivered all the time, networks wouldn't need us to fix them) :-) Does that make a 7500 with VIPs a L3 switch? A 12000 with distributed forwarding processors? -- it dependscall it (d)cef switching router if you want but i have to kinda agree with ken's comments. in my opinion the major difference between a tradition router and a l3 switch is the way packet switching takes place. in a tradition router the packet switching are done in software (microprocessor based), Big difference if the microprocessors (note plural) aren't doing anything except forwarding, and run a real time OS. The key thing is that you don't want forwarding going through the processor that runs routing protocols, system management, etc. A real challenge is where to implement QoS, because it tends to get beyond the complexity of a true ASIC and really has to be done in a microcode-loaded processor. whereas in l3 switch it is done by asic in hw and mls is used to increase routing performance by doing packet switching and rewrites in hw (asics). There's a bit of Cisco marketing-speak here, which was actually a reaction to competitors who brought up the concept switch if you can, route when you must. Hardware and software technology have moved on since then, and the line is much more blurred between the two. It's more important to think of separating the forwarding, control, and upper layer services path (and being sure there's no mutual interference) than it is to consider the actual hardware processing elements (ASICs, microcoded or RISC processors, etc.) This emphasis on ASICs also ignores a couple of common bottlenecks: memory and fabric. To some extent, you can get around memory limitations by having distributed memories for distributed processors. For the fabric, you can move from shared bus, to shared memory, and eventually to crossbar (ignoring optical trends). As I mentioned in a previous post that's partially below, you don't necessarily need ASICs if you have enough distributed processors, using the term processor to include microcode sequencers, FPGAs and EA-FPGAs, etc. In research prototypes, I've been involved in routers that had true processors, running on the forwarding boards, that ran a real-time OS. These processors did have certain functions custom-built in hardware. Also, the processors can have coprocessors -- the Nortel Shasta products, for example, have an encryption chip more or less next to general board-level processors, with a high-speed path between them. Even with ASICs, the L2 and L3 decisions, rewrite, etc. often are in separate chips. Remember a processor can be implemented as bit slices operating in a set of ICs. Substituting router for L3 switch is a good idea, but go farther than that. You can think of a high-performance router as a small hidden network, containing one or more (think high availability) path determination routing processors/hosts that download FIB information to multiple forwarding processors/hosts. One public and vendor-independent discussion of this architecture continues in the IETF FORCES Working Group (go to www.ietf.org and navigate to Working Groups). What does this mean to us? Not much other than for capacity planning. IMHO, an L3 switch has a longer life than a router. Not really, as you say in your next paragraph. I could go off into the ozone and say all high-speed routers are L3 switches. Indeed, ASICs aren't a necessity. I've worked on research router designs that used RISC processors in each forwarding and path determination engine, which gave lots of power but much more flexibility than ASICs. Admittedly, at least one of these was a specifically designed
RE: Does MLS (Layer 3 switching) require VLANs? [7:63147]
hi, comments in-line: -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Howard C. Berkowitz Sent: Wednesday, February 19, 2003 2:17 PM To: [EMAIL PROTECTED] Subject: RE: Does MLS (Layer 3 switching) require VLANs? [7:63147] At 6:51 PM + 2/19/03, Vicky Rode wrote: comments in-line: -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Howard C. Berkowitz Sent: Tuesday, February 18, 2003 6:42 AM To: [EMAIL PROTECTED] Subject: Re: Does MLS (Layer 3 switching) require VLANs? [7:63147] At 5:30 AM + 2/18/03, Ken Diliberto wrote: The nit I'm picking is inline... (I'm feeling like chipping in tonight) The Long and Winding Road 02/17/03 06:13PM [snip] if I have a 75xx router with 300 ethernet ports, and I bridge all those ports, do I have an L3 switch, or a router? [KD] You have a router performing L2 operations (forwarding, switching, bridging -- whatever). Would a cheap Linksys switch be faster? What makes a L3 switch in my mind is where the forwarding happens. If the L3 CPU (new way to look at it?) has to handle every packet, that's a router. If the first L3 packet is handled by the CPU which then programs ASICs to handle the rest of the flow without bothering the CPU, that's an L3 switch. Is there a difference from a packet/network perspective? No. The L2 headers and L3 headers are all properly updated in both cases (at least we *hope* they are) and traffic is delivered most of the time. (If it was delivered all the time, networks wouldn't need us to fix them) :-) Does that make a 7500 with VIPs a L3 switch? A 12000 with distributed forwarding processors? -- it dependscall it (d)cef switching router if you want but i have to kinda agree with ken's comments. in my opinion the major difference between a tradition router and a l3 switch is the way packet switching takes place. in a tradition router the packet switching are done in software (microprocessor based), Big difference if the microprocessors (note plural) aren't doing anything except forwarding, and run a real time OS. The key thing is that you don't want forwarding going through the processor that runs routing protocols, system management, etc. -vicky true enough. but in my opinion it depends on what hw you have in play and for what purpose. whether it is going to be classic line cards, switch fabric cards or distributed forwarding cards and whether the packet switching is going to be flow based or cef based. i guess one should have a good understanding for what their network traffic looks like and a good baseline before retrofitting to high powered hw which can be a big waste of money and resources. A real challenge is where to implement QoS, because it tends to get beyond the complexity of a true ASIC and really has to be done in a microcode-loaded processor. --vicky for me polling and gathering different qos snmp data variables has been a challenge rather than hw issue, so i can't really comment on that. whereas in l3 switch it is done by asic in hw and mls is used to increase routing performance by doing packet switching and rewrites in hw (asics). There's a bit of Cisco marketing-speak here, which was actually a reaction to competitors who brought up the concept switch if you can, route when you must. Hardware and software technology have moved on since then, and the line is much more blurred between the two. It's more important to think of separating the forwarding, control, and upper layer services path (and being sure there's no mutual interference) than it is to consider the actual hardware processing elements (ASICs, microcoded or RISC processors, etc.) ---vicky in my opinion, what's important and necessary is control/forward plane inter-relation. that's all. regards, /vicky This emphasis on ASICs also ignores a couple of common bottlenecks: memory and fabric. To some extent, you can get around memory limitations by having distributed memories for distributed processors. For the fabric, you can move from shared bus, to shared memory, and eventually to crossbar (ignoring optical trends). As I mentioned in a previous post that's partially below, you don't necessarily need ASICs if you have enough distributed processors, using the term processor to include microcode sequencers, FPGAs and EA-FPGAs, etc. In research prototypes, I've been involved in routers that had true processors, running on the forwarding boards, that ran a real-time OS. These processors did have certain functions custom-built in hardware. Also, the processors can have coprocessors -- the Nortel Shasta products, for example, have an encryption chip more or less next to general board-level processors, with a high-speed path between them. Even with ASICs, the L2 and L3 decisions, rewrite, etc. often are in separate chips. Remember a processor can
Re: Does MLS (Layer 3 switching) require VLANs? [7:63147]
I have a data center on the cisco campus that has well over 80 subnets in it, using L3 routing and no vlans on the 6509 gateways(routers).. We also have a production data center that uses 6509's with vlans that span different areas in the data center...due to the application structure of the servers and the fact that a lot of the servers have a need for redundant nics ... It works both ways folks...depends on what the need is Larry Letterman Network Engineer Cisco Systems - Original Message - From: Priscilla Oppenheimer To: Sent: Monday, February 17, 2003 10:07 PM Subject: RE: Does MLS (Layer 3 switching) require VLANs? [7:63147] I'm loath to continue this discussion, but I do have a question for Kelly. Why do you have a VLAN at all in your example?? Isn't a single VLAN sort of like one hand clapping? Seriously, what role is it playing in your network? Of course you don't have to have VLANs to do routing/L3 switching, as you probabaly know. But maybe there's some weird configuration gotcha, specific to the 6509? Just curious. Thanks. Larry said the majority of the Cisco campus is networked with L3 switches and not using vlans. That says a lot right there! Priscilla Kelly Cobean wrote: All, I'd like to add to this something that I haven't seen in other posts yet, and that is a quick look at layer2 function. I have a Catalyst 6509 with an MSFC on it. There is only *ONE* VLAN configured on the MSFC, however, that VLAN has several secondary addresses assigned to it (I know, not a great solution, but let's not go there). If I do a show mls entry on my switch, it is full of entries for hosts talking to hosts on the same VLAN. My point? When a host wants to talk to a host on another subnet (VLAN or not), it ANDs the address with it's own mask, determines that the host is in fact on a different subnet, then arps (if necessary) for it's default gateway (the MSFC) and sends the packet on it's way. The 6509/MSFC receive the packet and begin the MLS cache setup process (candidate packet, timeout, etc). All this is still done inspite of the fact that the MSFC only has a single VLAN. -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Stephen Hoover Sent: Monday, February 17, 2003 8:33 PM To: [EMAIL PROTECTED] Subject: Re: Does MLS (Layer 3 switching) require VLANs? [7:63147] - actually it is by doing secondaries, but i would highly recommend doing vlans if possible. keep it clean and simple. one may also configure the physical interfaces as L3 interfaces - just as one might do on a router with several ethernet ports. Oo ok, now THAT statement leads me to believe the L3 switching IS possible without VLANs. -Stephen [EMAIL PROTECTED] Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=63241t=63147 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: Does MLS (Layer 3 switching) require VLANs? [7:63147]
Vicky Rode shaped photons and electrons to say: see comments in-line: -Original Message- From: Stephen Hoover [mailto:[EMAIL PROTECTED]] Sent: Monday, February 17, 2003 11:20 AM To: Vicky Rode Cc: [EMAIL PROTECTED] Subject: Re: Does MLS (Layer 3 switching) require VLANs? [7:63147] Say for instance I have 2 hosts on the same layer 3 switch, but the two hosts are on 2 different IP subnets (No VLANs are defined). That's not possible! if you are talking about 2 IP subnet, than: - actually it is by doing secondaries, but i would highly recommend doing vlans if possible. keep it clean and simple. Vicki, You mention the use of secondary IP's. On a L3 switch (a switch with the router engine in it) is it not possible to define Ethernet sub interfaces instead of using secondary IPs - without VLANs defined? Yes and no. Secondaries and VLANs serve different purposes. Basic IP assumption:1 physical medium[1] = 1 subnet Secondary assumption: 1 physical medium[2] = multiple subnets Basic VLAN assumption: multiple phyical media [3] = 1 subnet VLAN with secondaries: multiple physical media = multiple subnets on all Notes - [1] Based on the local versus remote IP assumption: if a host is on your subnet, you have layer 2 connectivity to it. if a host is on a different subnet, you need to reach it through a router. This works nicely for broadcast and point-to-point media. NBMA and demand circuits break the local-vs-remote assumption. If you do assume a broadcast* medium, then the physical medium = 1 broadcast domain = 1 subnet (* broadcast is used loosely -- multicast is often closer. Some stupid NICs don't recognize multicasts and treat all multicasts as a broadcast. Broadcasts, indeed, are special cases of multicasts.) [2] The medium simultaneously must support a broadcast domain for each subnet, unless it is a non-broadcast medium. [3] The media in different locations are assumed to be linked by L2** trunking, typically IEEE 802.1q. While the trunks do contain traffic from multiple subnets, they are effectively tunneled. The only multicasts on the trunk medium are for layer management functions, such as 802.1d, 802.1q, VTP, etc. (** there are exotic variants where you could carry trunking over a conventionally routed tunnel, but let's not go there.) yes you can but when you create sub-interfaces it ask for encapsulation type and this is where vlans come into play. Encapsulation type is one reason to use VLANs, because it does create different broadcast domains for each encapsulation. This is preferred, but Cisco certainly has supported secondaries for different encapsulations -- more an IPX than an IP support technique. whereas with secondaries it will route between the subnets. I'm sorry to be so thick, I'm just not getting it. If a L3 switch (with a routing module/engine in it) is essentially a wire speed router, then the VLAN just seems like an additional identifier on top of the L3 address - and doesn't really serve any purpose. Not exactly. It lets you have the _same_ broadcast domain in several L2 switches. That's what gives you the portability of hosts from VLAN (same subnet) to same VLAN in different buildings. There need be only one router on the subnet, but there can be multiple VLAN segments connected by trunking. In my previous example, 2 hosts on the same L3 switch, but on 2 different IP subnets - wouldn't a defined Ethernet subinterface be each clients respective gateway, and thus normal L3 routing would occur, just at switch speeds - well let me you ask this, why not just supernet and put all stations on the same subnet (don't do this i'm being facetious). that's because you do not want to create this huge broadcast domain. that's the whole purpose of having vlans. if this still doesn't make sense, feel free to ask...would love to help. Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=63259t=63147 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: Does MLS (Layer 3 switching) require VLANs? [7:63147]
At 5:30 AM + 2/18/03, Ken Diliberto wrote: The nit I'm picking is inline... (I'm feeling like chipping in tonight) The Long and Winding Road 02/17/03 06:13PM [snip] if I have a 75xx router with 300 ethernet ports, and I bridge all those ports, do I have an L3 switch, or a router? [KD] You have a router performing L2 operations (forwarding, switching, bridging -- whatever). Would a cheap Linksys switch be faster? What makes a L3 switch in my mind is where the forwarding happens. If the L3 CPU (new way to look at it?) has to handle every packet, that's a router. If the first L3 packet is handled by the CPU which then programs ASICs to handle the rest of the flow without bothering the CPU, that's an L3 switch. Is there a difference from a packet/network perspective? No. The L2 headers and L3 headers are all properly updated in both cases (at least we *hope* they are) and traffic is delivered most of the time. (If it was delivered all the time, networks wouldn't need us to fix them) :-) Does that make a 7500 with VIPs a L3 switch? A 12000 with distributed forwarding processors? Substituting router for L3 switch is a good idea, but go farther than that. You can think of a high-performance router as a small hidden network, containing one or more (think high availability) path determination routing processors/hosts that download FIB information to multiple forwarding processors/hosts. One public and vendor-independent discussion of this architecture continues in the IETF FORCES Working Group (go to www.ietf.org and navigate to Working Groups). What does this mean to us? Not much other than for capacity planning. IMHO, an L3 switch has a longer life than a router. Not really, as you say in your next paragraph. I could go off into the ozone and say all high-speed routers are L3 switches. Indeed, ASICs aren't a necessity. I've worked on research router designs that used RISC processors in each forwarding and path determination engine, which gave lots of power but much more flexibility than ASICs. Admittedly, at least one of these was a specifically designed processor, but it definitely was software loadable and ran a real time OS. ASIC gets blurry anyway, when you start getting into the pure hard-etched IC, field-programmable gate arrays, electrically alterable field-programmable gate arrays, microcode sequencers, etc. When I design networks, I don't think L3 switch. I think about routers interconnecting L2 segments. I even draw them that way most of the time. :-) My advice to those having problems with this subject: Replace every occurrence of layer 3 switch with router. [/KD] Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=63260t=63147 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: Does MLS (Layer 3 switching) require VLANs? [7:63147]
Stephen Hoover wrote: Ken, Thanks for the input on this discussion. I follow and understand your example without any problems. Now if taking it back to the original original question - Does L3 switching require VLANs - produces this question for your example: You state 1 fiber feed for both Science and Engineering in the Labs building. I am then assuming that they are all connected to the same set of switches (Layer 2) in that building. Could you have not just simply assigned the hosts for Science to 1 IP network and the hosts for Engineering to another IP network - then created respective gateway interfaces for each network back on the common Layer 3 switch and accomplished the same thing?? It depends on the meaning of thing in your accomplish the same thing comment. :-) I think you already figured out your confusion and maybe this message is old, but I'll reply just in case. With your design you would accomplish connectivity. However, you would not accomplish separation of broadcast traffic for the two user communities. VLANs in the L2-switched part of the network give you that. VLANs have lots of features, but that's one of their primary ones. I think Ken's example is one of the cleanest I've seen. I may have to borrow it for my classes. Thanks for a good discussion, Stephen. THE END (hopefully! :-) Priscilla If the answer is yes, I will followup with another question. If the answer is no, then please explain. Thanks!! Stephen - Original Message - From: Ken Diliberto To: Sent: Tuesday, February 18, 2003 12:24 AM Subject: Re: Does MLS (Layer 3 switching) require VLANs? [7:63147] Stephen, You're getting there. Let me give an example of how VLANs are used (I'd draw a picture, but it probably wouldn't look good). For this example, let's use two of the colleges on my university network: Science and Engineering. Each has their own block of IP addresses and want their traffic separate from the other. They also want flat addressing (no subnetting). We have three buildings: Science, Engineering and Labs. Science and Engineering both have computer labs in the Labs building. Each want their labs on their respective IP address blocks. If money were no object, this would be fairly easy with vanilla switches and a router with two ethernet interfaces. Multiple fiber feeds and two sets of switches would be everywhere. With budget limitations (for this example), we only have a single fiber feed to each location. That means each fiber feed needs to carry traffic for both networks. To keep the traffic separate, we partition the switch ports into two LANs: LAN 10 and LAN 20. These two LANs in one switch are treated as unique. To do this, the switch creates Virtual LANs or VLANs. The fiber feeds are now trunks because a header is added to each frame to identify the VLAN it belongs to. So far so good? Why would we need a router? To talk between VLANs. Do routers understand trunks? Yes. This brings up one more concept: the Router on a Stick. A router on a stick is a router with a single network connection. This single connection is configured as a trunk so the router can see all the different VLANs. If the router finds a packet on VLAN 10 with a destination on VLAN 20, it rewrites the headers for the destination and puts it back on the same trunk with VLAN 20 headers. Remember: replace layer 3 switch with router every time you see it. That might make more sense. Hope this helps. Ken Stephen Hoover 02/17/03 06:55PM I appreciate everyone's input on this subject to help me understand this concept. As far as the newbies comment goes - I most definitely am. I'm about as green as they come. I have both my CCNA and my CCDA, but my only real experience is installing 2 T1s (at different locations) and configuring NAT for them. I have large amount of knowledge, just no experience. It has been my goal and my dream to become a serious network engineer for the last 6 years, but I just cannot seem to get a job that offers any experience. Everytime I get a network position, I just seemed to end up doing desktop support. When I first heard the term Layer 3 switching (some 4 years ago now) the first thing that popped into my mind was a switch that can route. I never even heard of a VLAN until a couple of years ago. The Cisco Study guide starts off talking about VLANs, and moves right into Inter-VLAN routing without ever really discussing Layer 3 switching as a seperate process. This is really where my confusion started. The book makes it sound like L3 switching is directly dependent on VLANs, and I just didn't see it - it wasn't something I was just willing to accept. Further more, the book states that VLANs allow for physical
Re: Does MLS (Layer 3 switching) require VLANs? [7:63147]
Stephen Hoover wrote: Ken, Thanks for the input on this discussion. I follow and understand your example without any problems. Now if taking it back to the original original question - Does L3 switching require VLANs - produces this question for your example: You state 1 fiber feed for both Science and Engineering in the Labs building. I am then assuming that they are all connected to the same set of switches (Layer 2) in that building. Could you have not just simply assigned the hosts for Science to 1 IP network and the hosts for Engineering to another IP network - then created respective gateway interfaces for each network back on the common Layer 3 switch and accomplished the same thing?? It depends on the meaning of thing in your accomplish the same thing comment. :-) I think you already figured out your confusion and maybe this message is old, but I'll reply just in case. With your design you would accomplish connectivity. However, you would not accomplish separation of broadcast traffic for the two user communities. VLANs in the L2-switched part of the network give you that. VLANs have lots of features, but that's one of their primary ones. I think Ken's example is one of the cleanest I've seen. I may have to borrow it for my classes. Thanks for a good discussion, Stephen. THE END (hopefully! :-) Priscilla If the answer is yes, I will followup with another question. If the answer is no, then please explain. Thanks!! Stephen - Original Message - From: Ken Diliberto To: Sent: Tuesday, February 18, 2003 12:24 AM Subject: Re: Does MLS (Layer 3 switching) require VLANs? [7:63147] Stephen, You're getting there. Let me give an example of how VLANs are used (I'd draw a picture, but it probably wouldn't look good). For this example, let's use two of the colleges on my university network: Science and Engineering. Each has their own block of IP addresses and want their traffic separate from the other. They also want flat addressing (no subnetting). We have three buildings: Science, Engineering and Labs. Science and Engineering both have computer labs in the Labs building. Each want their labs on their respective IP address blocks. If money were no object, this would be fairly easy with vanilla switches and a router with two ethernet interfaces. Multiple fiber feeds and two sets of switches would be everywhere. With budget limitations (for this example), we only have a single fiber feed to each location. That means each fiber feed needs to carry traffic for both networks. To keep the traffic separate, we partition the switch ports into two LANs: LAN 10 and LAN 20. These two LANs in one switch are treated as unique. To do this, the switch creates Virtual LANs or VLANs. The fiber feeds are now trunks because a header is added to each frame to identify the VLAN it belongs to. So far so good? Why would we need a router? To talk between VLANs. Do routers understand trunks? Yes. This brings up one more concept: the Router on a Stick. A router on a stick is a router with a single network connection. This single connection is configured as a trunk so the router can see all the different VLANs. If the router finds a packet on VLAN 10 with a destination on VLAN 20, it rewrites the headers for the destination and puts it back on the same trunk with VLAN 20 headers. Remember: replace layer 3 switch with router every time you see it. That might make more sense. Hope this helps. Ken Stephen Hoover 02/17/03 06:55PM I appreciate everyone's input on this subject to help me understand this concept. As far as the newbies comment goes - I most definitely am. I'm about as green as they come. I have both my CCNA and my CCDA, but my only real experience is installing 2 T1s (at different locations) and configuring NAT for them. I have large amount of knowledge, just no experience. It has been my goal and my dream to become a serious network engineer for the last 6 years, but I just cannot seem to get a job that offers any experience. Everytime I get a network position, I just seemed to end up doing desktop support. When I first heard the term Layer 3 switching (some 4 years ago now) the first thing that popped into my mind was a switch that can route. I never even heard of a VLAN until a couple of years ago. The Cisco Study guide starts off talking about VLANs, and moves right into Inter-VLAN routing without ever really discussing Layer 3 switching as a seperate process. This is really where my confusion started. The book makes it sound like L3 switching is directly dependent on VLANs, and I just didn't see it - it wasn't something I was just willing to accept. Further more, the book states that VLANs allow for physical
Re: Does MLS (Layer 3 switching) require VLANs? [7:63147]
Priscilla, All I want is credit. :-) Some guy on one of the many mailling lists I frequent put it this way: (maybe not) Ken Priscilla Oppenheimer 02/18/03 12:06PM [snip] I think Ken's example is one of the cleanest I've seen. I may have to borrow it for my classes. [snip] Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=63303t=63147 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: Does MLS (Layer 3 switching) require VLANs? [7:63147]
Ken Diliberto wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... Priscilla, All I want is credit. :-) if it makes you feel better, Ken, I always credit you with at least two cents worth I'm going to be visiting some of your compadres int the next couple of weeks. Dare I drop your name? ;- Some guy on one of the many mailling lists I frequent put it this way: (maybe not) Ken Priscilla Oppenheimer 02/18/03 12:06PM [snip] I think Ken's example is one of the cleanest I've seen. I may have to borrow it for my classes. [snip] Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=63305t=63147 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: Does MLS (Layer 3 switching) require VLANs? [7:63147]
Priscilla, Ok, you caught me not telling the whole truth. There is a second VLAN on the switch, but my point was that the MLS cache is full of entries for one host talking to another host off of the same VLAN interface but on a secondary subnet, indicating that L3 switching (routing) took place for that data-flow...So now I guess there are two hands clapping ;-) You sure do keep us all on our toes!!! Thanks! -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] Sent: Tuesday, February 18, 2003 1:08 AM To: [EMAIL PROTECTED] Subject: RE: Does MLS (Layer 3 switching) require VLANs? [7:63147] I'm loath to continue this discussion, but I do have a question for Kelly. Why do you have a VLAN at all in your example?? Isn't a single VLAN sort of like one hand clapping? Seriously, what role is it playing in your network? Of course you don't have to have VLANs to do routing/L3 switching, as you probabaly know. But maybe there's some weird configuration gotcha, specific to the 6509? Just curious. Thanks. Larry said the majority of the Cisco campus is networked with L3 switches and not using vlans. That says a lot right there! Priscilla Kelly Cobean wrote: All, I'd like to add to this something that I haven't seen in other posts yet, and that is a quick look at layer2 function. I have a Catalyst 6509 with an MSFC on it. There is only *ONE* VLAN configured on the MSFC, however, that VLAN has several secondary addresses assigned to it (I know, not a great solution, but let's not go there). If I do a show mls entry on my switch, it is full of entries for hosts talking to hosts on the same VLAN. My point? When a host wants to talk to a host on another subnet (VLAN or not), it ANDs the address with it's own mask, determines that the host is in fact on a different subnet, then arps (if necessary) for it's default gateway (the MSFC) and sends the packet on it's way. The 6509/MSFC receive the packet and begin the MLS cache setup process (candidate packet, timeout, etc). All this is still done inspite of the fact that the MSFC only has a single VLAN. -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Stephen Hoover Sent: Monday, February 17, 2003 8:33 PM To: [EMAIL PROTECTED] Subject: Re: Does MLS (Layer 3 switching) require VLANs? [7:63147] - actually it is by doing secondaries, but i would highly recommend doing vlans if possible. keep it clean and simple. one may also configure the physical interfaces as L3 interfaces - just as one might do on a router with several ethernet ports. Oo ok, now THAT statement leads me to believe the L3 switching IS possible without VLANs. -Stephen Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=63316t=63147 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: Does MLS (Layer 3 switching) require VLANs? [7:63147]
Howard, It would be so much fun to not understand some of this up close. :-) Howard C. Berkowitz 02/18/03 06:42AM [snip] Does that make a 7500 with VIPs a L3 switch? A 12000 with distributed forwarding processors? Substituting router for L3 switch is a good idea, but go farther than that. You can think of a high-performance router as a small hidden network, containing one or more (think high availability) path determination routing processors/hosts that download FIB information to multiple forwarding processors/hosts. One public and vendor-independent discussion of this architecture continues in the IETF FORCES Working Group (go to www.ietf.org and navigate to Working Groups). What does this mean to us? Not much other than for capacity planning. IMHO, an L3 switch has a longer life than a router. Not really, as you say in your next paragraph. I could go off into the ozone and say all high-speed routers are L3 switches. Indeed, ASICs aren't a necessity. I've worked on research router designs that used RISC processors in each forwarding and path determination engine, which gave lots of power but much more flexibility than ASICs. Admittedly, at least one of these was a specifically designed processor, but it definitely was software loadable and ran a real time OS. ASIC gets blurry anyway, when you start getting into the pure hard-etched IC, field-programmable gate arrays, electrically alterable field-programmable gate arrays, microcode sequencers, etc. [snip] Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=63323t=63147 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: Does MLS (Layer 3 switching) require VLANs? [7:63147]
DEar Stefen, you are doing a bit of confusion: so does layer 3 switching require the use of VLANs to actually do the switching? It's true the contrary case: the Vlans requires L3 to be routed, or, in other terms, to comunicate each others. The L3 switching has no sens without VLAN Say for instance I have 2 hosts on the same layer 3 switch, but the two hosts are on 2 different IP subnets (No VLANs are defined). That's not possible! if you are talking about 2 IP subnet, than: 1) you are talking about 2 subnet in 2 distinct sides of a router 2) you are talking about 2 Vlans in one L3 switch Host A wants to talk to host B. Can the switch not look up the routing info and then know to switch to that port? I am not seeing where the requirement for the VLAN comes into play. 1) host A and Host B are in two different VLAn: they need the L3 engine to comunicate 2) host A and host B are in the same Vlan but they have IP addresses (be careful this anyway a mistake!) who belongs to different VLAN: A can't comunicate with B because A doesn't know the MAC of B ... A can have knoledge of the MAC's of a) the hosts in the same subnet b) the gateway of the A's subnet and B's MAC doesn't match either of the a and b case. Hope this halp you Greetings Luca Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=63153t=63147 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: Does MLS (Layer 3 switching) require VLANs? [7:63147]
comment in-line: -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] Sent: Monday, February 17, 2003 2:10 AM To: [EMAIL PROTECTED] Subject: RE: Does MLS (Layer 3 switching) require VLANs? [7:63147] DEar Stefen, you are doing a bit of confusion: so does layer 3 switching require the use of VLANs to actually do the switching? It's true the contrary case: the Vlans requires L3 to be routed, or, in other terms, to comunicate each others. The L3 switching has no sens without VLAN Say for instance I have 2 hosts on the same layer 3 switch, but the two hosts are on 2 different IP subnets (No VLANs are defined). That's not possible! if you are talking about 2 IP subnet, than: - actually it is by doing secondaries, but i would highly recommend doing vlans if possible. keep it clean and simple. /vicky 1) you are talking about 2 subnet in 2 distinct sides of a router 2) you are talking about 2 Vlans in one L3 switch Host A wants to talk to host B. Can the switch not look up the routing info and then know to switch to that port? I am not seeing where the requirement for the VLAN comes into play. 1) host A and Host B are in two different VLAn: they need the L3 engine to comunicate 2) host A and host B are in the same Vlan but they have IP addresses (be careful this anyway a mistake!) who belongs to different VLAN: A can't comunicate with B because A doesn't know the MAC of B ... A can have knoledge of the MAC's of a) the hosts in the same subnet b) the gateway of the A's subnet and B's MAC doesn't match either of the a and b case. Hope this halp you Greetings Luca Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=63160t=63147 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: Does MLS (Layer 3 switching) require VLANs? [7:63147]
so does layer 3 switching require the use of VLANs to actually do the switching? It's true the contrary case: the Vlans requires L3 to be routed, or, in other terms, to comunicate each others. The L3 switching has no sens without VLAN Right, I understand that inter-VLAN routing requires L3 switching - but is the opposite also true? Does L3 switching require VLANs to be defined? If that is the case, then it would lead me to believe that L3 switching is based on VLAN info and not on the IP address, but I don't think that is correct. Thanks for the help! Stephen Hoover Dallas, Texas Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=63169t=63147 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: Does MLS (Layer 3 switching) require VLANs? [7:63147]
Say for instance I have 2 hosts on the same layer 3 switch, but the two hosts are on 2 different IP subnets (No VLANs are defined). That's not possible! if you are talking about 2 IP subnet, than: - actually it is by doing secondaries, but i would highly recommend doing vlans if possible. keep it clean and simple. Vicki, You mention the use of secondary IP's. On a L3 switch (a switch with the router engine in it) is it not possible to define Ethernet sub interfaces instead of using secondary IPs - without VLANs defined? I'm sorry to be so thick, I'm just not getting it. If a L3 switch (with a routing module/engine in it) is essentially a wire speed router, then the VLAN just seems like an additional identifier on top of the L3 address - and doesn't really serve any purpose. In my previous example, 2 hosts on the same L3 switch, but on 2 different IP subnets - wouldn't a defined Ethernet subinterface be each clients respective gateway, and thus normal L3 routing would occur, just at switch speeds Thanks again! Stephen Hoover Dallas, Texas Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=63171t=63147 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: Does MLS (Layer 3 switching) require VLANs? [7:63147]
Vicky Rode wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... comment in-line: -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] Sent: Monday, February 17, 2003 2:10 AM To: [EMAIL PROTECTED] Subject: RE: Does MLS (Layer 3 switching) require VLANs? [7:63147] DEar Stefen, you are doing a bit of confusion: so does layer 3 switching require the use of VLANs to actually do the switching? It's true the contrary case: the Vlans requires L3 to be routed, or, in other terms, to comunicate each others. The L3 switching has no sens without VLAN Say for instance I have 2 hosts on the same layer 3 switch, but the two hosts are on 2 different IP subnets (No VLANs are defined). That's not possible! if you are talking about 2 IP subnet, than: - actually it is by doing secondaries, but i would highly recommend doing vlans if possible. keep it clean and simple. one may also configure the physical interfaces as L3 interfaces - just as one might do on a router with several ethernet ports. /vicky 1) you are talking about 2 subnet in 2 distinct sides of a router 2) you are talking about 2 Vlans in one L3 switch Host A wants to talk to host B. Can the switch not look up the routing info and then know to switch to that port? I am not seeing where the requirement for the VLAN comes into play. 1) host A and Host B are in two different VLAn: they need the L3 engine to comunicate 2) host A and host B are in the same Vlan but they have IP addresses (be careful this anyway a mistake!) who belongs to different VLAN: A can't comunicate with B because A doesn't know the MAC of B ... A can have knoledge of the MAC's of a) the hosts in the same subnet b) the gateway of the A's subnet and B's MAC doesn't match either of the a and b case. Hope this halp you Greetings Luca Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=63186t=63147 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: Does MLS (Layer 3 switching) require VLANs? [7:63147]
I've been following this thread, and have offered a comment or two along the way. Perhaps I should offer some thoughts here at the source. note that I have not read any of the exam study materials in question, so I don't know what is or is not being stated in the courseware. I can offer that just because it says so in the study materials doesn't mean that's the way it is. comments below Stephen Hoover wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... I am studying for the CCNP Switching exam and it covers VLANs and layer 3 switching moderately. It states that Cisco recommends a 1 to 1 mapping of VLANs to subnets. It also states that VLANs can be used to break up broadcast domains. this is a reasonable, simple approach, and thus one that appeals to my reasonably simple mind. When you create different subnets, you are already breaking up broadcast domains, so does layer 3 switching require the use of VLANs to actually do the switching? this is where the confusion, no doubt introduced by the marketing people, set in. suppose you have a router with three ethernet interfaces, and each of these interfaces is plugged into a different hub ( no switch ) hosts on each of these hubs are in the same broadcast domain ( same collision domain too, but I digress ) hosts in each of these domains cannot reach hosts ( or servers ) in other domians, on different hubs, without routing. this would be true, even if you had all hosts on the same great big hub with 500 ports. You could have hosts on the same hub, but having different L3 ( IP ) addresses. communication between hosts on different subnets, even if they are on the same hub, require the intrercession of a router. vlans, made possible by various 802.1 specifications, are really just a way of expressing logical broadcast domains. layer 3 switching is really routing. an L3 switch has the routing function built into it, rather than using a separate piece of equipment. Say for instance I have 2 hosts on the same layer 3 switch, but the two hosts are on 2 different IP subnets (No VLANs are defined). Host A wants to talk to host B. Can the switch not look up the routing info and then know to switch to that port? I am not seeing where the requirement for the VLAN comes into play. despite what others have said, you can do this. it is wasteful, in that a host plugged into an L3 port would require 4 ip addresses because you have a subnet with two hosts ( the PC and the port, and the net number and the broadcast address ). whereas if you have a vlan, that vlan is a virutal port that represents the physical ports as a single subnet to the L3 ( routing ) function. If VLANs are required for layer 3 switching, is that pretty much standard across the industry, or that a Cisco only thing? forget this L3 switch versus router distinction. it is confusing, and misrepresentational. think instead in terms of how traffic moves through a network. think instead of a vlan as a virtual logical construct that represents one or more ports as a single broadcast domain to a router. it doesn't matter that the router is integrated into the switch hardware with an ASIC and code, or is an external device. HTH Thanks! Stephen Hoover Dallas, Texas Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=63190t=63147 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: Does MLS (Layer 3 switching) require VLANs? [7:63147]
This might help. What does the V stand for in VLAN? Virtual. VLANs are a method for emulating Real LANs in a switched network. The original poster seems disillusioned with VLANs. Well, I am too. :-) You can't do much with them that you can't do with a bunch of Real LANs connected by routers. First we had hubs and bridges and routers. Then switches came out. They were cheaper and faster than routers, so everyone jumped on the bandwagon and started designing huge flat networks with mostly switches and maybe one router to get out to the rest of the world. Ah, but there was a problem! A L2 switch forwards broadcasts out all ports. And this was in the mid-1990s when PC CPUs were slow as molasses and got bogged down by broadcasts and multicasts. Dreadful protocols like SAP and RTMP and NetBIOS were rampant! Something had to be done. So, hu, should we go back to designing our networks with routers, which don't forward broadcasts? Nah, still too expensive. Better come up with a way to emulate LAN and IP subnet benefits on a switched networks. OK, let's invent VLANs! But how do the VLANs talk to each other? Oh dear, we better go back to routers. Nah, still too slow, though it will work in a pinch. I know! We could speed them up and call them L3 switches. One last rather serious comment. This is not a comment on the newbiness of the original poster, but I must say that I think it is common for newbies to get confused by VLANs. Cisco teaches VLANs without ever teaching basic networking 101. People can't understand VLANs unless they first understand a lot more about protocol behavior and traffic flow. VLANs are really an advanced topic and shouldn't be covered so early on in the Cisco test progression. Either that or CCNA should be beefed up to teach something useful, if you ask me, which they didn't. Priscilla The Long and Winding Road wrote: I've been following this thread, and have offered a comment or two along the way. Perhaps I should offer some thoughts here at the source. note that I have not read any of the exam study materials in question, so I don't know what is or is not being stated in the courseware. I can offer that just because it says so in the study materials doesn't mean that's the way it is. comments below Stephen Hoover wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... I am studying for the CCNP Switching exam and it covers VLANs and layer 3 switching moderately. It states that Cisco recommends a 1 to 1 mapping of VLANs to subnets. It also states that VLANs can be used to break up broadcast domains. this is a reasonable, simple approach, and thus one that appeals to my reasonably simple mind. When you create different subnets, you are already breaking up broadcast domains, so does layer 3 switching require the use of VLANs to actually do the switching? this is where the confusion, no doubt introduced by the marketing people, set in. suppose you have a router with three ethernet interfaces, and each of these interfaces is plugged into a different hub ( no switch ) hosts on each of these hubs are in the same broadcast domain ( same collision domain too, but I digress ) hosts in each of these domains cannot reach hosts ( or servers ) in other domians, on different hubs, without routing. this would be true, even if you had all hosts on the same great big hub with 500 ports. You could have hosts on the same hub, but having different L3 ( IP ) addresses. communication between hosts on different subnets, even if they are on the same hub, require the intrercession of a router. vlans, made possible by various 802.1 specifications, are really just a way of expressing logical broadcast domains. layer 3 switching is really routing. an L3 switch has the routing function built into it, rather than using a separate piece of equipment. Say for instance I have 2 hosts on the same layer 3 switch, but the two hosts are on 2 different IP subnets (No VLANs are defined). Host A wants to talk to host B. Can the switch not look up the routing info and then know to switch to that port? I am not seeing where the requirement for the VLAN comes into play. despite what others have said, you can do this. it is wasteful, in that a host plugged into an L3 port would require 4 ip addresses because you have a subnet with two hosts ( the PC and the port, and the net number and the broadcast address ). whereas if you have a vlan, that vlan is a virutal port that represents the physical ports as a single subnet to the L3 ( routing ) function. If VLANs are required for layer 3 switching, is that pretty much standard across the industry, or that a Cisco only thing? forget this L3 switch versus router distinction. it is confusing, and misrepresentational. think instead in terms of how traffic moves through a network. think instead of a vlan as a virtual
Re: Does MLS (Layer 3 switching) require VLANs? [7:63147]
good for you, Cil. This discussion was ( and still is, to judge from my in-box ) filled with misdirection and poor information. Cisco and all the other vendors are absolutely to blame for this. a router is a function, not a device so is a switch. what does it matter where the function resides, or how it is accomplished? -- TANSTAAFL there ain't no such thing as a free lunch Priscilla Oppenheimer wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... This might help. What does the V stand for in VLAN? Virtual. VLANs are a method for emulating Real LANs in a switched network. The original poster seems disillusioned with VLANs. Well, I am too. :-) You can't do much with them that you can't do with a bunch of Real LANs connected by routers. First we had hubs and bridges and routers. Then switches came out. They were cheaper and faster than routers, so everyone jumped on the bandwagon and started designing huge flat networks with mostly switches and maybe one router to get out to the rest of the world. Ah, but there was a problem! A L2 switch forwards broadcasts out all ports. And this was in the mid-1990s when PC CPUs were slow as molasses and got bogged down by broadcasts and multicasts. Dreadful protocols like SAP and RTMP and NetBIOS were rampant! Something had to be done. So, hu, should we go back to designing our networks with routers, which don't forward broadcasts? Nah, still too expensive. Better come up with a way to emulate LAN and IP subnet benefits on a switched networks. OK, let's invent VLANs! But how do the VLANs talk to each other? Oh dear, we better go back to routers. Nah, still too slow, though it will work in a pinch. I know! We could speed them up and call them L3 switches. One last rather serious comment. This is not a comment on the newbiness of the original poster, but I must say that I think it is common for newbies to get confused by VLANs. Cisco teaches VLANs without ever teaching basic networking 101. People can't understand VLANs unless they first understand a lot more about protocol behavior and traffic flow. VLANs are really an advanced topic and shouldn't be covered so early on in the Cisco test progression. Either that or CCNA should be beefed up to teach something useful, if you ask me, which they didn't. Priscilla The Long and Winding Road wrote: I've been following this thread, and have offered a comment or two along the way. Perhaps I should offer some thoughts here at the source. note that I have not read any of the exam study materials in question, so I don't know what is or is not being stated in the courseware. I can offer that just because it says so in the study materials doesn't mean that's the way it is. comments below Stephen Hoover wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... I am studying for the CCNP Switching exam and it covers VLANs and layer 3 switching moderately. It states that Cisco recommends a 1 to 1 mapping of VLANs to subnets. It also states that VLANs can be used to break up broadcast domains. this is a reasonable, simple approach, and thus one that appeals to my reasonably simple mind. When you create different subnets, you are already breaking up broadcast domains, so does layer 3 switching require the use of VLANs to actually do the switching? this is where the confusion, no doubt introduced by the marketing people, set in. suppose you have a router with three ethernet interfaces, and each of these interfaces is plugged into a different hub ( no switch ) hosts on each of these hubs are in the same broadcast domain ( same collision domain too, but I digress ) hosts in each of these domains cannot reach hosts ( or servers ) in other domians, on different hubs, without routing. this would be true, even if you had all hosts on the same great big hub with 500 ports. You could have hosts on the same hub, but having different L3 ( IP ) addresses. communication between hosts on different subnets, even if they are on the same hub, require the intrercession of a router. vlans, made possible by various 802.1 specifications, are really just a way of expressing logical broadcast domains. layer 3 switching is really routing. an L3 switch has the routing function built into it, rather than using a separate piece of equipment. Say for instance I have 2 hosts on the same layer 3 switch, but the two hosts are on 2 different IP subnets (No VLANs are defined). Host A wants to talk to host B. Can the switch not look up the routing info and then know to switch to that port? I am not seeing where the requirement for the VLAN comes into play. despite what others have said, you can do this. it is wasteful, in that a host plugged into an L3 port would require 4 ip addresses because you have a subnet
Re: Does MLS (Layer 3 switching) require VLANs? [7:63147]
- actually it is by doing secondaries, but i would highly recommend doing vlans if possible. keep it clean and simple. one may also configure the physical interfaces as L3 interfaces - just as one might do on a router with several ethernet ports. Oo ok, now THAT statement leads me to believe the L3 switching IS possible without VLANs. -Stephen Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=63204t=63147 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: Does MLS (Layer 3 switching) require VLANs? [7:63147]
hi, comments in-line: -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of The Long and Winding Road Sent: Monday, February 17, 2003 3:41 PM To: [EMAIL PROTECTED] Subject: Re: Does MLS (Layer 3 switching) require VLANs? [7:63147] Vicky Rode wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... comment in-line: -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] Sent: Monday, February 17, 2003 2:10 AM To: [EMAIL PROTECTED] Subject: RE: Does MLS (Layer 3 switching) require VLANs? [7:63147] DEar Stefen, you are doing a bit of confusion: so does layer 3 switching require the use of VLANs to actually do the switching? It's true the contrary case: the Vlans requires L3 to be routed, or, in other terms, to comunicate each others. The L3 switching has no sens without VLAN Say for instance I have 2 hosts on the same layer 3 switch, but the two hosts are on 2 different IP subnets (No VLANs are defined). That's not possible! if you are talking about 2 IP subnet, than: - actually it is by doing secondaries, but i would highly recommend doing vlans if possible. keep it clean and simple. one may also configure the physical interfaces as L3 interfaces - just as one might do on a router with several ethernet ports. --- true enough.i would love to move to native ios provided it reaches complete feature parity w/ catos. that's all. regards, /vicky /vicky 1) you are talking about 2 subnet in 2 distinct sides of a router 2) you are talking about 2 Vlans in one L3 switch Host A wants to talk to host B. Can the switch not look up the routing info and then know to switch to that port? I am not seeing where the requirement for the VLAN comes into play. 1) host A and Host B are in two different VLAn: they need the L3 engine to comunicate 2) host A and host B are in the same Vlan but they have IP addresses (be careful this anyway a mistake!) who belongs to different VLAN: A can't comunicate with B because A doesn't know the MAC of B ... A can have knoledge of the MAC's of a) the hosts in the same subnet b) the gateway of the A's subnet and B's MAC doesn't match either of the a and b case. Hope this halp you Greetings Luca Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=63205t=63147 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: Does MLS (Layer 3 switching) require VLANs? [7:63147]
Stephen Hoover wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... - actually it is by doing secondaries, but i would highly recommend doing vlans if possible. keep it clean and simple. one may also configure the physical interfaces as L3 interfaces - just as one might do on a router with several ethernet ports. Oo ok, now THAT statement leads me to believe the L3 switching IS possible without VLANs. forgive the rant. you are not to blame. all the marketing hype is to blame. forget OSI. For L-anything. for data ( packets, frames, whatever ) to get from here to there, somethng has to happen. if I have a 75xx router with 300 ethernet ports, and I bridge all those ports, do I have an L3 switch, or a router? for data to get from here to there, it must be forwarded. I know Howard is going to jump all over my fast and loose use of the term forward but that is what happens. If my PC wants to send data to your PC, that data is forwarded to your PC. If your PC and mine are on the same subnet / hub / switch / vlan, it is L2 forwarding ( switching ). If the devices are on different subnets / switches . vlans / hubs then the packets are L3 forwarded ( routed ) As Priscilla has been pointing out, the issue is one of how networks work, how packets are forwarded, how data gets from here to there. An L3 device is a router is able to forward packets based on an L3 address, whether that L3 address be appletalk, IPX, or IP. an L2 device is a switch is a bridge is able to forward packets based on L2 addreses i.e. MAC address. the fact that some equipment can function as both a switch and a router ( anyone remember brouters? ) is irrelevant. on a 3550, a physical port ( into which you plug the ethernet patch cable ) can be stand alone physical, can be part of a vlan, thus making it distinct from ports on the same box that are not in the same vlan, or can have an IP ( L3 ) address. an SVI ( switch virtual interface ), invoked by the command interface vlan x, is a representation of a group of ports that have been placed into a single vlan. The SVI represents those ports to the routing funtion, and behaves no differently that a router's ethernet port plugged into a hub. I'm hoping this helps clarify the concept. I believe you have been confused by the study materials you are reading, and by the mis-information that has been presented here on the list. sorry to have not taken the time to be more thorough in earlier replies. you can never go wrong studying Priscilla's posts, either. hope this is starting to make sense to you . Chuck -Stephen Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=63206t=63147 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: Does MLS (Layer 3 switching) require VLANs? [7:63147]
HTH, Thank you for these comments - this clears up a lot of confusion for me. To sum, just to make sure I really have this: Layer 3 switching is possible without VLANs (however the opposite is not true. Well at least not without some form of Layer 3 intervention.) VLANs simply the administration behind Layer 3 switching design. Physical location (port location) independence is ok in front of the layer 3 switch that is the the hosts gateway. Up to the hosts distribution switch. VLANs extending beyond the distribution layer switch across the core is generally not a good idea - possible, but not recommended. This is the flat earth design that Priscilla mentioned - VLANs that extend across the entire internetwork. Thanks! Stephen Hoover Dallas, Texas - Original Message - From: The Long and Winding Road To: Sent: Monday, February 17, 2003 6:00 PM Subject: Re: Does MLS (Layer 3 switching) require VLANs? [7:63147] I've been following this thread, and have offered a comment or two along the way. Perhaps I should offer some thoughts here at the source. note that I have not read any of the exam study materials in question, so I don't know what is or is not being stated in the courseware. I can offer that just because it says so in the study materials doesn't mean that's the way it is. comments below Stephen Hoover wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... I am studying for the CCNP Switching exam and it covers VLANs and layer 3 switching moderately. It states that Cisco recommends a 1 to 1 mapping of VLANs to subnets. It also states that VLANs can be used to break up broadcast domains. this is a reasonable, simple approach, and thus one that appeals to my reasonably simple mind. When you create different subnets, you are already breaking up broadcast domains, so does layer 3 switching require the use of VLANs to actually do the switching? this is where the confusion, no doubt introduced by the marketing people, set in. suppose you have a router with three ethernet interfaces, and each of these interfaces is plugged into a different hub ( no switch ) hosts on each of these hubs are in the same broadcast domain ( same collision domain too, but I digress ) hosts in each of these domains cannot reach hosts ( or servers ) in other domians, on different hubs, without routing. this would be true, even if you had all hosts on the same great big hub with 500 ports. You could have hosts on the same hub, but having different L3 ( IP ) addresses. communication between hosts on different subnets, even if they are on the same hub, require the intrercession of a router. vlans, made possible by various 802.1 specifications, are really just a way of expressing logical broadcast domains. layer 3 switching is really routing. an L3 switch has the routing function built into it, rather than using a separate piece of equipment. Say for instance I have 2 hosts on the same layer 3 switch, but the two hosts are on 2 different IP subnets (No VLANs are defined). Host A wants to talk to host B. Can the switch not look up the routing info and then know to switch to that port? I am not seeing where the requirement for the VLAN comes into play. despite what others have said, you can do this. it is wasteful, in that a host plugged into an L3 port would require 4 ip addresses because you have a subnet with two hosts ( the PC and the port, and the net number and the broadcast address ). whereas if you have a vlan, that vlan is a virutal port that represents the physical ports as a single subnet to the L3 ( routing ) function. If VLANs are required for layer 3 switching, is that pretty much standard across the industry, or that a Cisco only thing? forget this L3 switch versus router distinction. it is confusing, and misrepresentational. think instead in terms of how traffic moves through a network. think instead of a vlan as a virtual logical construct that represents one or more ports as a single broadcast domain to a router. it doesn't matter that the router is integrated into the switch hardware with an ASIC and code, or is an external device. HTH Thanks! Stephen Hoover Dallas, Texas Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=63208t=63147 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: Does MLS (Layer 3 switching) require VLANs? [7:63147]
I appreciate everyone's input on this subject to help me understand this concept. As far as the newbies comment goes - I most definitely am. I'm about as green as they come. I have both my CCNA and my CCDA, but my only real experience is installing 2 T1s (at different locations) and configuring NAT for them. I have large amount of knowledge, just no experience. It has been my goal and my dream to become a serious network engineer for the last 6 years, but I just cannot seem to get a job that offers any experience. Everytime I get a network position, I just seemed to end up doing desktop support. When I first heard the term Layer 3 switching (some 4 years ago now) the first thing that popped into my mind was a switch that can route. I never even heard of a VLAN until a couple of years ago. The Cisco Study guide starts off talking about VLANs, and moves right into Inter-VLAN routing without ever really discussing Layer 3 switching as a seperate process. This is really where my confusion started. The book makes it sound like L3 switching is directly dependent on VLANs, and I just didn't see it - it wasn't something I was just willing to accept. Further more, the book states that VLANs allow for physical location independence, but is also says that VLANs should not cross the core - those 2 statements seem partly contradictory to me. Here is a summary of how I see VLANs now. Layer 3 switching is possible without VLANs (however the opposite is not true. Well at least not without some form of Layer 3 intervention.) VLANs simply the administration behind Layer 3 switching design. Physical location (port location) independence is ok in front of the layer 3 switch that is the the hosts gateway. Up to the hosts distribution switch. VLANs extending beyond the distribution layer switch across the core is generally not a good idea - possible, but not recommended. This is the flat earth design that Priscilla mentioned - VLANs that extend across the entire internetwork. Thanks! Stephen Hoover Dallas, Texas - Original Message - From: Priscilla Oppenheimer To: Sent: Monday, February 17, 2003 7:04 PM Subject: Re: Does MLS (Layer 3 switching) require VLANs? [7:63147] This might help. What does the V stand for in VLAN? Virtual. VLANs are a method for emulating Real LANs in a switched network. The original poster seems disillusioned with VLANs. Well, I am too. :-) You can't do much with them that you can't do with a bunch of Real LANs connected by routers. Better come up with a way to emulate LAN and IP subnet benefits on a switched networks. OK, let's invent VLANs! But how do the VLANs talk to each other? Oh dear, we better go back to routers. Nah, still too slow, though it will work in a pinch. I know! We could speed them up and call them L3 switches. One last rather serious comment. This is not a comment on the newbiness of the original poster, but I must say that I think it is common for newbies to get confused by VLANs. Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=63210t=63147 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: Does MLS (Layer 3 switching) require VLANs? [7:63147]
its entirely possible without vlans..majority of the cisco campus is networked with layer 3 switches and not using vlans Larry Letterman Network Engineer Cisco Systems - Original Message - From: Stephen Hoover To: Sent: Monday, February 17, 2003 5:32 PM Subject: Re: Does MLS (Layer 3 switching) require VLANs? [7:63147] - actually it is by doing secondaries, but i would highly recommend doing vlans if possible. keep it clean and simple. one may also configure the physical interfaces as L3 interfaces - just as one might do on a router with several ethernet ports. Oo ok, now THAT statement leads me to believe the L3 switching IS possible without VLANs. -Stephen [EMAIL PROTECTED] Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=63216t=63147 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: Does MLS (Layer 3 switching) require VLANs? [7:63147]
All, I'd like to add to this something that I haven't seen in other posts yet, and that is a quick look at layer2 function. I have a Catalyst 6509 with an MSFC on it. There is only *ONE* VLAN configured on the MSFC, however, that VLAN has several secondary addresses assigned to it (I know, not a great solution, but let's not go there). If I do a show mls entry on my switch, it is full of entries for hosts talking to hosts on the same VLAN. My point? When a host wants to talk to a host on another subnet (VLAN or not), it ANDs the address with it's own mask, determines that the host is in fact on a different subnet, then arps (if necessary) for it's default gateway (the MSFC) and sends the packet on it's way. The 6509/MSFC receive the packet and begin the MLS cache setup process (candidate packet, timeout, etc). All this is still done inspite of the fact that the MSFC only has a single VLAN. -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Stephen Hoover Sent: Monday, February 17, 2003 8:33 PM To: [EMAIL PROTECTED] Subject: Re: Does MLS (Layer 3 switching) require VLANs? [7:63147] - actually it is by doing secondaries, but i would highly recommend doing vlans if possible. keep it clean and simple. one may also configure the physical interfaces as L3 interfaces - just as one might do on a router with several ethernet ports. Oo ok, now THAT statement leads me to believe the L3 switching IS possible without VLANs. -Stephen Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=63221t=63147 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: Does MLS (Layer 3 switching) require VLANs? [7:63147]
hi stephen, see comments in-line: -Original Message- From: Stephen Hoover [mailto:[EMAIL PROTECTED]] Sent: Monday, February 17, 2003 11:20 AM To: Vicky Rode Cc: [EMAIL PROTECTED] Subject: Re: Does MLS (Layer 3 switching) require VLANs? [7:63147] Say for instance I have 2 hosts on the same layer 3 switch, but the two hosts are on 2 different IP subnets (No VLANs are defined). That's not possible! if you are talking about 2 IP subnet, than: - actually it is by doing secondaries, but i would highly recommend doing vlans if possible. keep it clean and simple. Vicki, You mention the use of secondary IP's. On a L3 switch (a switch with the router engine in it) is it not possible to define Ethernet sub interfaces instead of using secondary IPs - without VLANs defined? yes you can but when you create sub-interfaces it ask for encapsulation type and this is where vlans come into play. whereas with secondaries it will route between the subnets. I'm sorry to be so thick, I'm just not getting it. If a L3 switch (with a routing module/engine in it) is essentially a wire speed router, then the VLAN just seems like an additional identifier on top of the L3 address - and doesn't really serve any purpose. In my previous example, 2 hosts on the same L3 switch, but on 2 different IP subnets - wouldn't a defined Ethernet subinterface be each clients respective gateway, and thus normal L3 routing would occur, just at switch speeds - well let me you ask this, why not just supernet and put all stations on the same subnet (don't do this i'm being facetious). that's because you do not want to create this huge broadcast domain. that's the whole purpose of having vlans. if this still doesn't make sense, feel free to ask...would love to help. regards, /vicky Thanks again! Stephen Hoover Dallas, Texas Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=63228t=63147 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: Does MLS (Layer 3 switching) require VLANs? [7:63147]
- actually it is by doing secondaries, but i would highly recommend doing vlans if possible. keep it clean and simple. one may also configure the physical interfaces as L3 interfaces - just as one might do on a router with several ethernet ports. Oo ok, now THAT statement leads me to believe the L3 switching IS possible without VLANs. Ouch. L3 switching is routing. Routing interconnects subnets/IP prefixes. If a VLAN is a subnet, it can be routed. If a piece of wire is a subnet, it can be routed. Again: L3 switching is not a technology. It is salesbabble. Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=63229t=63147 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: Does MLS (Layer 3 switching) require VLANs? [7:63147]
The nit I'm picking is inline... (I'm feeling like chipping in tonight) The Long and Winding Road 02/17/03 06:13PM [snip] if I have a 75xx router with 300 ethernet ports, and I bridge all those ports, do I have an L3 switch, or a router? [KD] You have a router performing L2 operations (forwarding, switching, bridging -- whatever). Would a cheap Linksys switch be faster? What makes a L3 switch in my mind is where the forwarding happens. If the L3 CPU (new way to look at it?) has to handle every packet, that's a router. If the first L3 packet is handled by the CPU which then programs ASICs to handle the rest of the flow without bothering the CPU, that's an L3 switch. Is there a difference from a packet/network perspective? No. The L2 headers and L3 headers are all properly updated in both cases (at least we *hope* they are) and traffic is delivered most of the time. (If it was delivered all the time, networks wouldn't need us to fix them) :-) What does this mean to us? Not much other than for capacity planning. IMHO, an L3 switch has a longer life than a router. When I design networks, I don't think L3 switch. I think about routers interconnecting L2 segments. I even draw them that way most of the time. :-) My advice to those having problems with this subject: Replace every occurrence of layer 3 switch with router. [/KD] Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=63230t=63147 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: Does MLS (Layer 3 switching) require VLANs? [7:63147]
I'm loath to continue this discussion, but I do have a question for Kelly. Why do you have a VLAN at all in your example?? Isn't a single VLAN sort of like one hand clapping? Seriously, what role is it playing in your network? Of course you don't have to have VLANs to do routing/L3 switching, as you probabaly know. But maybe there's some weird configuration gotcha, specific to the 6509? Just curious. Thanks. Larry said the majority of the Cisco campus is networked with L3 switches and not using vlans. That says a lot right there! Priscilla Kelly Cobean wrote: All, I'd like to add to this something that I haven't seen in other posts yet, and that is a quick look at layer2 function. I have a Catalyst 6509 with an MSFC on it. There is only *ONE* VLAN configured on the MSFC, however, that VLAN has several secondary addresses assigned to it (I know, not a great solution, but let's not go there). If I do a show mls entry on my switch, it is full of entries for hosts talking to hosts on the same VLAN. My point? When a host wants to talk to a host on another subnet (VLAN or not), it ANDs the address with it's own mask, determines that the host is in fact on a different subnet, then arps (if necessary) for it's default gateway (the MSFC) and sends the packet on it's way. The 6509/MSFC receive the packet and begin the MLS cache setup process (candidate packet, timeout, etc). All this is still done inspite of the fact that the MSFC only has a single VLAN. -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Stephen Hoover Sent: Monday, February 17, 2003 8:33 PM To: [EMAIL PROTECTED] Subject: Re: Does MLS (Layer 3 switching) require VLANs? [7:63147] - actually it is by doing secondaries, but i would highly recommend doing vlans if possible. keep it clean and simple. one may also configure the physical interfaces as L3 interfaces - just as one might do on a router with several ethernet ports. Oo ok, now THAT statement leads me to believe the L3 switching IS possible without VLANs. -Stephen Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=63233t=63147 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: Does MLS (Layer 3 switching) require VLANs? [7:63147]
Priscilla Oppenheimer wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... I'm loath to continue this discussion, but I do have a question for Kelly. Why do you have a VLAN at all in your example?? Isn't a single VLAN sort of like one hand clapping? Seriously, what role is it playing in your network? said half seriously, isn't a network with NO vlans no different than a network with ONE vlan? ;- Of course you don't have to have VLANs to do routing/L3 switching, as you probabaly know. But maybe there's some weird configuration gotcha, specific to the 6509? Just curious. Thanks. Larry said the majority of the Cisco campus is networked with L3 switches and not using vlans. That says a lot right there! Priscilla Kelly Cobean wrote: All, I'd like to add to this something that I haven't seen in other posts yet, and that is a quick look at layer2 function. I have a Catalyst 6509 with an MSFC on it. There is only *ONE* VLAN configured on the MSFC, however, that VLAN has several secondary addresses assigned to it (I know, not a great solution, but let's not go there). If I do a show mls entry on my switch, it is full of entries for hosts talking to hosts on the same VLAN. My point? When a host wants to talk to a host on another subnet (VLAN or not), it ANDs the address with it's own mask, determines that the host is in fact on a different subnet, then arps (if necessary) for it's default gateway (the MSFC) and sends the packet on it's way. The 6509/MSFC receive the packet and begin the MLS cache setup process (candidate packet, timeout, etc). All this is still done inspite of the fact that the MSFC only has a single VLAN. -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Stephen Hoover Sent: Monday, February 17, 2003 8:33 PM To: [EMAIL PROTECTED] Subject: Re: Does MLS (Layer 3 switching) require VLANs? [7:63147] - actually it is by doing secondaries, but i would highly recommend doing vlans if possible. keep it clean and simple. one may also configure the physical interfaces as L3 interfaces - just as one might do on a router with several ethernet ports. Oo ok, now THAT statement leads me to believe the L3 switching IS possible without VLANs. -Stephen Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=63235t=63147 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: Does MLS (Layer 3 switching) require VLANs? [7:63147]
Ken, Thanks for the input on this discussion. I follow and understand your example without any problems. Now if taking it back to the original original question - Does L3 switching require VLANs - produces this question for your example: You state 1 fiber feed for both Science and Engineering in the Labs building. I am then assuming that they are all connected to the same set of switches (Layer 2) in that building. Could you have not just simply assigned the hosts for Science to 1 IP network and the hosts for Engineering to another IP network - then created respective gateway interfaces for each network back on the common Layer 3 switch and accomplished the same thing?? If the answer is yes, I will followup with another question. If the answer is no, then please explain. Thanks!! Stephen - Original Message - From: Ken Diliberto To: Sent: Tuesday, February 18, 2003 12:24 AM Subject: Re: Does MLS (Layer 3 switching) require VLANs? [7:63147] Stephen, You're getting there. Let me give an example of how VLANs are used (I'd draw a picture, but it probably wouldn't look good). For this example, let's use two of the colleges on my university network: Science and Engineering. Each has their own block of IP addresses and want their traffic separate from the other. They also want flat addressing (no subnetting). We have three buildings: Science, Engineering and Labs. Science and Engineering both have computer labs in the Labs building. Each want their labs on their respective IP address blocks. If money were no object, this would be fairly easy with vanilla switches and a router with two ethernet interfaces. Multiple fiber feeds and two sets of switches would be everywhere. With budget limitations (for this example), we only have a single fiber feed to each location. That means each fiber feed needs to carry traffic for both networks. To keep the traffic separate, we partition the switch ports into two LANs: LAN 10 and LAN 20. These two LANs in one switch are treated as unique. To do this, the switch creates Virtual LANs or VLANs. The fiber feeds are now trunks because a header is added to each frame to identify the VLAN it belongs to. So far so good? Why would we need a router? To talk between VLANs. Do routers understand trunks? Yes. This brings up one more concept: the Router on a Stick. A router on a stick is a router with a single network connection. This single connection is configured as a trunk so the router can see all the different VLANs. If the router finds a packet on VLAN 10 with a destination on VLAN 20, it rewrites the headers for the destination and puts it back on the same trunk with VLAN 20 headers. Remember: replace layer 3 switch with router every time you see it. That might make more sense. Hope this helps. Ken Stephen Hoover 02/17/03 06:55PM I appreciate everyone's input on this subject to help me understand this concept. As far as the newbies comment goes - I most definitely am. I'm about as green as they come. I have both my CCNA and my CCDA, but my only real experience is installing 2 T1s (at different locations) and configuring NAT for them. I have large amount of knowledge, just no experience. It has been my goal and my dream to become a serious network engineer for the last 6 years, but I just cannot seem to get a job that offers any experience. Everytime I get a network position, I just seemed to end up doing desktop support. When I first heard the term Layer 3 switching (some 4 years ago now) the first thing that popped into my mind was a switch that can route. I never even heard of a VLAN until a couple of years ago. The Cisco Study guide starts off talking about VLANs, and moves right into Inter-VLAN routing without ever really discussing Layer 3 switching as a seperate process. This is really where my confusion started. The book makes it sound like L3 switching is directly dependent on VLANs, and I just didn't see it - it wasn't something I was just willing to accept. Further more, the book states that VLANs allow for physical location independence, but is also says that VLANs should not cross the core - those 2 statements seem partly contradictory to me. Here is a summary of how I see VLANs now. Layer 3 switching is possible without VLANs (however the opposite is not true. Well at least not without some form of Layer 3 intervention.) VLANs simply the administration behind Layer 3 switching design. Physical location (port location) independence is ok in front of the layer 3 switch that is the the hosts gateway. Up to the hosts distribution switch. VLANs extending beyond the distribution layer switch across the core is generally not a good idea - possible, but not recommended. This is the flat earth design that Priscilla mentioned - VLANs that extend across the entire internetwork. Thanks! Stephen Hoover
Does MLS (Layer 3 switching) require VLANs? [7:63147]
I am studying for the CCNP Switching exam and it covers VLANs and layer 3 switching moderately. It states that Cisco recommends a 1 to 1 mapping of VLANs to subnets. It also states that VLANs can be used to break up broadcast domains. When you create different subnets, you are already breaking up broadcast domains, so does layer 3 switching require the use of VLANs to actually do the switching? Say for instance I have 2 hosts on the same layer 3 switch, but the two hosts are on 2 different IP subnets (No VLANs are defined). Host A wants to talk to host B. Can the switch not look up the routing info and then know to switch to that port? I am not seeing where the requirement for the VLAN comes into play. If VLANs are required for layer 3 switching, is that pretty much standard across the industry, or that a Cisco only thing? Thanks! Stephen Hoover Dallas, Texas Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=63147t=63147 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
