mutlilayer switches

Unanswered Question
Aug 23rd, 2007

Hi all, I did my bcmsn course last year, I remember them saying that switches are faster at routing than routers as its done in hardware, surely if they have a routing table this is done in software ? and so why are they faster ?

I have this problem too.
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avmabe Thu, 08/23/2007 - 05:52

My personal opinion is L3 switches are not faster than routers, they are the same. In my case we use GSRs and 7600's and they all to routing in hardware b/c they keep a table in an ASIC for routing. Same on switches by them moving packets L2 or L3 via ASICs.

jwdoherty Thu, 08/23/2007 - 05:55

Multilayer switches or switching routers, L2/L3, do perform routing forwarding using additional special hardware. They can forward L3 packets nearly or as quickly as a switch can forward L2 frames.

These new high speed L3 devices have also negated much of the old saying "switch when you can, route when you must". For instance, designs now use high speed L3 devices for a core's backbone where, in the past, L2 switches were used.

Mohamed Sobair Sun, 08/26/2007 - 01:35

Hi,

Yes this is right, Switches are faster in process because of the following:

For a router to perform normal routing, its going through the routing lookup , finding next hop for the destination Network, and resolve the next-hop mac address, this causes the overhead of the normal routing, untill CISCO introduced CEF , which builds its own table to faster the new process (Fast Switching Technology).

With a MLS which capable of a L3 task, the normal 1st process is routing (Normal Routing lookup), but all following processes is Switched for all packet have to be routed.

Regards,

Mohamed Sobair

rajatsetia Sun, 08/26/2007 - 20:28

Hi,

This is nowdays a very frquent query about difference between present days Multilayer switches and routers.

I have asked the same question at 2006 networker forum from cisco expert but could not get the exact answer.

I am referring to one of very good discussion on the similar topic, you will enjoy this :)

http://forums.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Network%20Infrastructure&topic=WAN%2C%20Routing%20and%20Switching&CommCmd=MB%3Fcmd%3Ddisplay_location%26location%3D.1ddf8a0e

HTH

rgds

avmabe Mon, 08/27/2007 - 04:59

The link you posted above was answered perfectly by a cisco employee.

The 6500 and 7600 have the exact same architecture. They have the same backplane and route/switch exactly the same.

jorgenolla Mon, 08/27/2007 - 05:14

Just my two cents:

L3 switches using CEF, use the concept of route once switch many. What this means is that the switch will look at the L3 address and if it has no information regarding that address in it's FIB and Adjacency table, it will then move the request to the routing processor.

Once that route is established, and entry will be made in the FIB and Adjacency tables; and there will be no more need for routing for that specific address.

After the L3 address is found in the FIB and Adjacency tables, the switch will work pretty much the same way it works with the ARP table, at L2 in hardware.

Best Regards

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