A layer 3 switch, as a previous poster said, is a switch, that has a route
processor (or that can connect to an external router), to perform Layer 3
functions "at wire speed". But it also has ASICs that performs other
functions that a normal Layer 2 switch doesn't have (as you'll see below).
"At wire speed" is a fancy way of waying "as fast as a switch normally
switches traffic".
In the below section, MLS-SE = Multilayer Switching Engine and MLS-RP =
Multilayer Route Processor.
This is not a mystery nor is it some special voodoo..... it's actually an
ingenious process....
1) A frame (ethernet let's say) containing an IP packet comes into a switch
port
2) The MLS-SE actually looks at the Layer 3 information (source/destination
IPs and port numbers) and looks in the MLS cache to see if there is already
an entry for this "flow" (a flow is a single unidirectional conversation
between the source/destination IPs and port #s)
3) If there is not an MLS cache entry, the MLS-SE makes a partial MLS cache
entry using the source/destination IP and port #s and sends the packet to
the MLS-RP (which could be an external router also)
4) The MLS-RP then routes the packet normally and hands the packet with the
rewritten Layer 2 info back to the MLS-SE.
5) The MLS-SE looks in the CAM table to find the port to reach the
destination MAC, uses that information to complete the MLS cache entry with
the proper switch port number, and forwards the packet out the destination
switch port.
Now for every subsequent packet in that flow (with the same
source/destination IP and port info), the MLS-SE (in step 2 above) will find
the MLS cache entry, rewrite the Layer 2 info, just as if the packet had
been routed by the MLS-RP, and spits it out the destination port. A Layer 3
switch differs from a regular Layer 2 switch (or even a router) because it
can perfom this "MLS cache lookup and Layer 2 rewrite" on the fly without
delaying the packet anymore than a normal switch, so therefore it's called
wirespeed. The NetFlow Feature Card (NFFC) or NFFC 2 is the hardware that
performs this MLS cache and flow comparison.
Keep in mind, that a flow is unidirectional. So if 2 computers are
communicating back and forth, the switch will actually have 2 MLS entries (2
flows), one for the conversation going in each direction.
The easy way to remember how a Layer 3 switch (Multilayer switch) works is:
"Route once, switch many"
It's actually a smoking idea that allows switches to take a great burden
from the routers having to route EVERY SINGLE PACKET, as well as giving the
appearance that this "routing" (which is truly being bypassed after the
first packet) is happening at wirespeed. Kudos to whomever thought this up!
Here's a great explanation of Layer 3 switching, flows, etc from Cisco:
http://www.cisco.com/univercd/cc/td/doc/product/lan/cat5000/rel_5_2/layer3/m
ls.htm
Mike W.
"Denton, Jason" wrote in message
[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
> Can anyone tell me what the REAL difference is between a layer3 switch and
a
> router?
>
> Jason
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=7434&t=7406
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