DeVoe, Charles (PKI) wrote: > > The decision to send to the RP isn't really based on the VLAN > is it?? > > I believe that the decision to send to the Routing Processor > (RP) is indeed > based on the VLAN. When a host wishes to communicate with > another host, the > IP on the host determines if the host is on the same subnet or > not. If it > is on the same subnet, it will send an ARP broadcast to > determine the MAC > address of the destination host. If the host is on another > subnet, the ARP > request will be for that of the default gateway (aka RP). The > RP will strip > out the source and destination MAC address and replace the > destination with > that of host B (or next hop) and place its own MAC address in > the source > address. The IP addresses will not be changed.
You jumped from the ARP to the actual packet. You also jumped to a conclusion that the switch knows to send the packet to the RP because of the VLAN? (That was the language I was disputing). The switch knows because of the destination MAC address. Also, and this is the most critical, you skipped the step whereby the RP knows which interface it should use for sending the packet and which destination MAC address it should use. It knows that because it looks in its routing table or cache for the destination IP address in the packet. That allows it to figure out the outgoing interface and send directly to the recipient or the next hop. In other words it does Layer 3 switching, aka routing, forwarding, relaying. It bases its forwarding decision on the IP address. Networking 101. Extremely silly to be arguing about this. Here's what happens: The RP responds to the ARP from the host that is trying to find a MAC address for its default gateway, which is configured as an IP address. (We're assuming the mapping of the default gateway IP address to MAC address is no longer in the host's ARP table. If it were, the ARP wouldn't happen.) The host then sends the actual packet that resulted in it needing to send an ARP in the first place. It puts the default gateway's MAC address in the MAC destination field. It puts the end station's IP address in the IP destination field. So the RP (which we're assuming is the default gateway) takes in the frame, strips off the MAC header and looks at the destination IP address to determine how to send the frame. In other words, it does Layer 3 "switching," sometimes also called relaying, forwarding, or routing. If necessary, it will ARP to get the MAC address of the final destination (if it's local) or next-hop destination. > > Switches and Bridges make decisions based on MAC address (layer > 2). Routers > make decisions based on IP address (layer 3). > > So is the RP making routing decisions based on the MAC > address??? No, of course not. Priscilla > > > > > > -----Original Message----- > From: Priscilla Oppenheimer [mailto:[EMAIL PROTECTED] > Sent: Tuesday, February 25, 2003 1:33 PM > To: [EMAIL PROTECTED] > Subject: Re: L3 Switching Huh???? [7:63728] > > > Robert Edmonds wrote: > > > > Layer 3 switching combines the best of switching and routing > in > > one > > platform. The main advantage here is speed. The way it works > > is, in a > > switch you have some kind of layer 3 routing engine (aka route > > processor, or > > RP). For example, the MSFC2 (Multilayer Switch Feature Card > 2) > > is one of > > the options available for the Cisco 6500 (and a couple of > > others, I think) > > switches. When the switch receives a packet bound for a > > different VLAN, it > > sends it to the RP. > > The decision to send to the RP isn't really based on the VLAN > is it?? The > decision is based on the MAC destination address, I would > assume. > > A host in VLAN 1 wants to send to a host in VLAN 2. Because > VLANs generally > equate to IP subnets, the host knows that it must send to its > default > gateway, which is the RP. It ARPs for the RP and gets a MAC > address. It > sends the frame then with the destinaton MAC address set to the > RP's > address. > > The L2 switch looks just at MAC addresses. That's what makes it > L2. It has > learned that this MAC address belongs to the RP. (Learning the > location of > MAC addresses is a basic L2 function). > > Now the RP can do L3 "switching." It looks at the IP > destination address to > determine where to send the frame. That's what makes it L3 > (i.e. that it > uses a L3 address for its decision). > > As far as switching, routing, forwarding, they all mean the > same thing. As > Kevin Banifaz said in one of the best, most concise answers > that we have > seen, "Switching is the function of directing frames or packets > from one > port or interface to another." > > Someone said that switching isn't a technical term. What a > shame. It > certainly used to be a good engineering term. Network equipment > developers > borrowed the term from our forefathers and foremathers who > worked on the > high-tech equipment of the 1800s and 1900s. Railroad tracks > switch trains. > Electrical equipment switches current. Telephone equipment > switches voice > conversations. Bridges, switches, and routers switch frames. > > I must direct you all, once again it seems, to Webster's > definition of > switch, the noun: > > Main Entry: 1switch > Pronunciation: 'swich > Function: noun > Etymology: perhaps from Middle Dutch swijch twig > Date: 1592 > 1 : a slender flexible whip, rod, or twig > 2 : an act of switching : as a : a blow with a switch b : a > shift from one > to another c : a change from the usual > 3 : a tuft of long hairs at the end of the tail of an animal > (as a cow) -- > see COW illustration > 4 a : a device made usually of two movable rails and necessary > connections > and designed to turn a locomotive or train from one track to > another b : a > railroad siding > 5 : a device for making, breaking, or changing the connections > in an > electrical circuit > 6 : a heavy strand of hair used in addition to a person's own > hair for some > coiffures > > If anyone else brings up this question, we may need to have > definition 1 > applied to them. Or, if we're nice, we'll use definition 6 on > your behind > instead. Or we'll say that you are definition 3. :-) > > Priscilla > > > > The RP makes the routing decision and puts > > an entry in > > the route cache for the switch. The first packet in a flow is > > routed and > > the rest are switched at wire speed, hence the increase in > > speed. That's > > kind of a simplified view, but I think it gets the general > idea > > across. So, > > layer 3 switching is both routing and switching, but faster > > (usually, > > anyway). > > > > ""DeVoe, Charles (PKI)"" wrote in > > message > > news:[EMAIL PROTECTED] > > > I am under the impression that switching is a layer 2 > > function and that > > > routing is a layer 3 function. I have seen several > > discussions talking > > > about layer 3 switching. Could someone explain this to me? > > Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=63797&t=63728 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
