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Webcast-Catalyst9k
New Member

Which router for handling GRE tunnels

I need a router that will have just one FastE interface but will have about 30 GRE/Tunnel interfaces. This router will mainly be routing from FastE to tunnel and vice-versa. I was comparing the 1721 to the 1751 and found something interesting. The 1721 has faster process switching speed than the 1751 but lower Fast/CEF switching numbers. In my case (FastE to Tunnel) which would apply? Process or Fast/CEF? If Fast/CEF is the preferred method, how do I turn this on? How do I apply it to my Tunnel interfaces?

Thanks,

Diego

3 REPLIES
Silver

Re: Which router for handling GRE tunnels

Diego,

Fast Switching is on by default "ip route-cache". To enable CEF "ip cef" globally will enable this on all supported interfaces.

Hope this is what you were looking for.

Don

New Member

Re: Which router for handling GRE tunnels

Last question would be: Is CEF used when routing from GRE tunnel interfaces to FastE interfaces? I ask this because I believe there are certain situations where CEF and/or fast switching are not applicable.

Thanks,

Diego

Silver

Re: Which router for handling GRE tunnels

Diego,

You are correct. If there is no valid adjacency to your destination network, for example, the packets would be punted to the processor for switching. CEF is supported on Tunnel interfaces. You can verify your switching patch by "show int stat" to ensure that the correct switching path is being used. Here is a snip from a defect that involved "no adjacency" and the workaround.

When there are no FIB entries for routes via a GRE tunnel

using Policy routing to set the default interface to that GRE tunnel, packets

forwarded through the interface are being punted to the process switch layer this can

be confirmed by looking at the interface switching stats via

'show interface stats'. The reason for this behaviour is that CEF has no adjacency

information for the Tunnel interface 'show adjacency tunnel'

Workaround: Add static route pointing out the GRE tunnel this will force

CEF to add the adjacency.

Hope this helps you,

Don

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