Is Route Cache Policy Needed on Cat3750?

Answered Question
Sep 21st, 2009

I recently implemented a policy routing setup on a Cat3750 stack in our network. Since the switch handles a high volume of traffic, I wanted to be sure to add 'ip route-cache policy' to the interfaces where policy routing is applied, in order to fast-switch the policy packets.

Well, policy routing is enabled and working, but when I issue the 'show ip cache policy' command, nothing shows up.

I can only conclude one of two things:

1) Policy routing is not being fast-switched

or

2) The policy routing info is loaded into ASICs and will be hardware switched. Thus, the command 'ip route-cache policy' may only be applicable to software routers and not layer 3 switches such as the Cat3750.

I have noted similar behavior on a Cat4510R switch, with SupV. Policy routing is also working, but nothing shows up in the output of 'show ip cache policy'.

Can anyone confirm whether #2 is in fact the way this is operating? The documentation implies that it is needed, but I suspect that it was written for a software router, and not a layer 3 switch.

Thank you,

Ron Buchalski

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Correct Answer by Giuseppe Larosa about 7 years 2 months ago

Hello Ron,

your understanding is correct:

multilayer switches are able to implement PBR by modifiying the action field in the TCAM tables where a pointer to the PBR next-hop can be placed instead of the "natural" destination based IP next-hop.

You can see the TCAM as a generalization of the CAM table of a L2 switch that allows to handle L3 switched flows.

This allows for an efficient implementation of PBR with no performance penalties but with some limitations on the possible set and match commands.

Hope to help

Giuseppe

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Correct Answer
Giuseppe Larosa Mon, 09/21/2009 - 10:41

Hello Ron,

your understanding is correct:

multilayer switches are able to implement PBR by modifiying the action field in the TCAM tables where a pointer to the PBR next-hop can be placed instead of the "natural" destination based IP next-hop.

You can see the TCAM as a generalization of the CAM table of a L2 switch that allows to handle L3 switched flows.

This allows for an efficient implementation of PBR with no performance penalties but with some limitations on the possible set and match commands.

Hope to help

Giuseppe

ronbuchalski Mon, 09/21/2009 - 10:43

Guiseppe,

Thank you for your quick reply. So, a followup question (which may be obvious)...is the command 'ip route-cache policy' not required on the interfaces where pbr is implemented?

Thanks again,

-rb

Giuseppe Larosa Mon, 09/21/2009 - 10:52

Hello Ron,

>> is the command 'ip route-cache policy' not required on the interfaces where pbr is implemented?

I do think so on multilayer switches the command should be meaningless.

It shouldn't make any difference

Hope to help

Giuseppe

ronbuchalski Mon, 09/21/2009 - 11:24

Guiseppe,

I was thinking the same thing, but just wanted to be sure about it before I removed it from the switch.

Thanks again,

-rb

Joseph W. Doherty Mon, 09/21/2009 - 14:49

Unsure it applies to all Cisco devices, but recall reading somewhere CEF eliminates the need for policy caching.

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