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Community Member

Set clause interpretation with Multi-VRF selection using PBR

I posted this on the LAN, Switching and Routing list, but I did not get any takers....

I am trying to figure out the semantic/operational differences between some of the set clauses used with Multi-VRF selection using Policy Based Routing (PBR).

Specifically, I am trying to figure what the difference is between this:

route-map Applied-to-a-VRF-Interface permit 100

match ip address 100

set global

set ip next-hop 192.168.0.1

and this:

route-map Applied-to-a-VRF-Interface permit 100

match ip address 100

set ip global next-hop 192.168.0.1

where:

interface Gig5/2

ip vrf forwarding NewVRF

ip address 192.168.1.1 255.255.255.0

ip policy route-map Applied-to-a-VRF-Interface

I've read the documentation found here http://www.cisco.com/en/US/docs/ios/12_2sr/12_2srb/feature/guide/srb2mvrf.html#wp1053934, but I don't quite follow the semantic differences between the "set global" clause followed by a "set ip next-hop" clause and the "set ip global next-hop" clause.

I have a particular application on a 6509 running 12.2(33)SXH3 whereby when I use the "set ip global next-hop" clause my traffic is getting software-switched but when I do a "set global / set ip next-hop" on different lines that the traffic is getting hardware switched. I just want to make sure I understand the difference with how these clauses are interpreted by the 6509 hardware.

I'm assuming that using the "set vrf" and "set ip vrf" syntax is conceptually the same as in the "global" instance.

Any ideas?

Clarke Morledge

College of William and Mary

1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Silver

Re: Set clause interpretation with Multi-VRF selection using PBR

Hello Clarke,

from the document you have linked:

set global-Routes packets through the global routing table. This command is useful when you want to route ingress packets belonging to a specific VRF through the global routing table.

•set ip global-Routes packets through the global routing table, where the next-hop lookup will be in the global routing table.

You have probably found an implementation issue: the command in two lines is easier to map on C6509 HW.

the set global says that in any case the packets will be routed to the global routing table.

the second command might require to verify availability of that next hop in the global routing table.

It could mean that if the specified next-hop is not available the packets have to be routed within the VRF table.

So the two lines formulation can be more efficiently implemented in TCAM tables.

Hope to help

Giuseppe

6 REPLIES
Hall of Fame Super Silver

Re: Set clause interpretation with Multi-VRF selection using PBR

Hello Clarke,

from the document you have linked:

set global-Routes packets through the global routing table. This command is useful when you want to route ingress packets belonging to a specific VRF through the global routing table.

•set ip global-Routes packets through the global routing table, where the next-hop lookup will be in the global routing table.

You have probably found an implementation issue: the command in two lines is easier to map on C6509 HW.

the set global says that in any case the packets will be routed to the global routing table.

the second command might require to verify availability of that next hop in the global routing table.

It could mean that if the specified next-hop is not available the packets have to be routed within the VRF table.

So the two lines formulation can be more efficiently implemented in TCAM tables.

Hope to help

Giuseppe

Community Member

Re: Set clause interpretation with Multi-VRF selection using PBR

Guiseppe,

I agree with your explanation, except that I'm still puzzled as to why I am getting software switched since the "next hop" in question has the route to the next hop available in the VRF.

There are a lot mysteries as to how Cisco programs these TCAMs. I just do not find the documentation to be particularly enlightening.

Thanks.

Clarke Morledge

Hall of Fame Super Silver

Re: Set clause interpretation with Multi-VRF selection using PBR

Hello Clarke,

thanks but actually you solved the issue by yourself !

and yes in some cases we can just take note of the observed behaviour in these complex systems.

Best Regards

Giuseppe

Re: Set clause interpretation with Multi-VRF selection using PBR

hi Clarke,

Out of curiosity, how did you know when the traffic was getting software-switched or hardware switched

Lejoe

Community Member

Re: Set clause interpretation with Multi-VRF selection using PBR

Lejoe,

Initially, I saw the problem right away. Typing on the router console got V-E-R-Y S-L-O-W.

show proc cpu sorted

also showed me that "IP Input" was the leader in grabbing the CPU. As soon as I pulled off the route-map from being applied, the CPU problem was corrected.

Generally, if my CPU utilization jumps up after I configured something, that's the best indication that I somehow got traffic to be software-switched instead of being handled by the TCAM.

Clarke

Re: Set clause interpretation with Multi-VRF selection using PBR

Hi Clarke,

Thanks for that. Did you issue a debug ip policy and see the difference for both those configurations for traffic generated from access-list 100.

My thoughts are

set global

set ip next-hop

it's successfully able to route the packet using the first statement and not even reaching the final statement. However with set global ip next-hop, it's explicitly forced to choose the next-hop.

http://www.ciscosystems.com/en/US/docs/ios/mpls/configuration/guide/mp_mltvrf_slct_pbr.html#wp1053974

HTH

Lejoe

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