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=63762&t=63728 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
