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, hummmm, 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 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=7&i=63198&t=63147 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
