Yes. The switch examines past the normal layer 2/MAC info to the IP layer 3
information, to find, depending on the mode, the destination and source IP
addresses and sometimes the port numbers (layer 4).
I recently posted a (rather) lengthy post outlining the process of
multilayer switching.... I can e-mail you a copy if you let me know
Basically, here's a quick overview. (for reference a "flow" is unidirection
traffic from a given source IP and port to a give destination IP and port)
When a frame arrives, it has a source MAC of the host that sent the frame
and a destination MAC of the IP gateway (which is the router interface).
The switch looks past the layer 2 headers at the source/destination IP and
port #s and checks in the Mulilayer Switching (MLS) cache. If there is not
entry matching this info, it makes a partial entry using the
source/destination IP and port #s and sends the frame to the router. The
router routes the packet and, in the process, rewrites the layer 2 info now
showing the source MAC as the outgoing interface on the router and the
destination MAC as the MAC of the destination host and sends it back to the
switch. The switch then examines the source/destination IP and port #s, and
sees the partial cache entry for this flow, and takes note of the new
"rewritten" source/destination MAC addresses and which switchport it will
exit and that completes the MLS cache entry.
For every frame in that flow after that, when the switch examines the
source/destination IP and port #s, it sees the MLS cache entry for that
flow, takes the MAC info that it saw from the first "routed" packet, and
simply rewrites the layer 2 info itself just as if the packet has been sent
through the router, and sends it out itself without every passing it to the
router.
This is a very basic summary of how this works because there are difference
kinds of flows, etc....... but this should give you a good idea of what
they mean when they say "The frame is rewritten and sent to the exit
interface....."
Mike W.
"Larry trav" wrote in message
[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
> Newbie question
>
> If a device is using the "fast switching" method and only the first packet
> is handled* by the router, does the switch look any deeper than the frame
> level to determine the switching for the remaining packets/frames? How
does
> it read the rewritten frame to know that it should send the remaing
packets
> of the session to the same switch port as the first frame without going
> through the router?
>
> Cisco site info
> *"The frame is rewritten and sent to the exit interface that services the
> destination. Subsequent packets for the same destination use the same
> switching path"
>
> TIA
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=7869&t=7867
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