That is a great answer!! That is above and beyond the call of duty and I am
printing this and keeping it for future ref.
Seriously. That is exactly why I like this group.
Thanks,
Jennifer Cribbs
6/6/2001 5:09:37 PM, "Michael L. Williams" wrote:
>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
Have a great day!!
Jennifer
Message Posted at:
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