MLS (Multilayer Switching)

Unanswered Question
Oct 2nd, 2007

I just looked at an implementation of a router on a stick that supposedly will not be a bottleneck because of the MLS between the router (7200) and the switch (3750). Does anyone know if the MLS works in that configuration?

I have been doing some research on the MLS feature. All I can find is that it was mainly a feature for the Cat5000 and Cat6000 running CatOS.

I have this problem too.
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glen.grant Tue, 10/02/2007 - 13:41

I don't think you can do this with a 3750 . You used to be able to do this with the old catos boxes if they had the correct hardware installed and the 7200 was configured correctly. With the speeds of the hardware these days I think this would be less of a concern than it used to be. If you are worried about speed the 3750 can do the routing itself and you could point like the default route to the 7200 . Not knowing your setup its hard to speculate on anything else.

Gilberto Olivar... Wed, 10/03/2007 - 06:27

I always understoood that MLS was a term related to a feature inside a switch. Meaning that a switch could do InterVLAN routing. The deployment that I saw involves a Cat3750 with VLANs (no SVIs) and one trunk interface connected to a 7200 series router doing the InterVLAN routing. The switch is just doing Layer2 functions. The idea of the implementation is that the 7200 will communicate with the 3750 using MLSP so that not all traffic goes through the trunk interface, but is switched (in the switch) instead of routed. The Cisco documentation mentions something like that for a Cat5500 and Cat6x000 using an MSFC, and also says that it is posible using an external router, but I haven't found an example using an external router, or documentation saying that the 3750 can do MLSP. The MLS commands available for the 3750 seemed to be related to qos.

I see what you're talking about, its been a while since I've looked at that. The 3750 isn't going to exactly what you're talking about. The good thing is that it will do L3 switching. All you have to do is create the SVIs for the VLANs, bind an ip address to them and, assuming "ip routing" (global config mode) is enabled you're on your way. You don't issue any "mls" commands unless you're doing something with QoS, as you've discovered.

Now there are something things you can't do with the 3750 (e.g. NAT) but for those things you can leave the SVI unnumbered and have that traffic forwarded by the router.

paul.matthews Wed, 10/03/2007 - 07:10

You do not need the external router to do MLS on the 3750. Configure it for routing and it just does it.

The router on a stick MLS approach what for the "early days" where a switch did not have the software or hardware to make L3 decisions. The idea was that the first packet in a flow would be passed up to a router, and the router would tell the switch what to do next time it saw one. This really went back to 6000s before the MSFC or MSM(spit), or the 5000s where you had a sup3, no RSM and already had a decent router.

Attaching a 7200 to a 3750 and having the 7200 make all the inter vlan decisions will not be the most efficient way.

It may be that the link to the 7200 is am L3 link and the 3750 is doing all the inter VLAN routing.



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