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Re: L3 Switching Huh???? [7:63728] posted 02/25/2003
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At 04:46 PM 2/25/2003 +0000, 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 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,

One should keep in mind that many vendors including Cisco have been capable 
of doing per packet routing at wire speed for some time and thus this 
"advantage" is a legacy attribute.

>""DeVoe, Charles (PKI)""  wrote in message
> > 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?

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