From: Howard C. Berkowitz (hcb@gettcomm.com)
Date: Mon May 17 2004 - 19:39:17 GMT-3
At 6:25 PM -0400 5/17/04, CCIE 2004 wrote:
>Hi Howard,
>
>
>
>Thank you for your response. However I had another question. Per your response
>
>
>
>"Bridges and layer 2 switches make decisions on _destination_ MAC
>
>addresses. If VLANs are used, layer 2 switches may make their first
>
>decisions, on a trunk port, on the VLAN ID field."
>
>
>Could you please confirm as from whatever I have seen and read it
>always says that Bridges and Switches build their tables based on
>the source mac address. I am not sure what you were referring to and
>I might have interpreted it the wrong way.
OK, I think I see the confusion. When a frame arrives on a port, if
the source address hasn't been seen before, it becomes associated
with that port. The next time a frame comes in destined for the
address, the frame will look up the destination address in the table
constructed from source addresses. The real-time decision forwarding
decision is made on the destination address, but the control plane
learning to build the table is indeed based on source address.
>
>
>
>
>Also, I was trying to find out how others would define Layer 3
>switching and differentiate it with traditional Layer 2 switching so
>that I could gain a better understanding of what it typically means
>or is it just marketing hype and a layer 3 switch is basically a
>layer 2 switch with say either a RSM like the Catalyst 5000s or a
>MSFC like for the catalyst 6000. They are traditionally layer 2
>switches with just routing functionality.
>
It's marketing hype. A layer 3 switch, for all practical purposes,
is a router with lots of LAN interfaces. It may be able to make
forwarding decisions based on layer 2 information, but the speed
difference between a L2 and L3 lookup, with modern implementations,
is trivial.
Consider the 12000 is officially the Gigabit Switch Router, and
you'll see how meaningless the term "switch" becomes.
-- =============================================================================== "What Problem are you trying to solve?" ***send Cisco questions to the list, so all can benefit -- not directly to me*** ******************************************************************************** Howard C. Berkowitz presentation downloads by anonymous ftp from www.netcases.net.
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