Route-cache/CEF on router WAN interfaces

[rajesh444]

Hi,

When both ip route-cache and CEF are enabled on a router's serial interface does one method take precedence over the other? In other words, I see entries for destination networks on the outputs of both "show ip cache" & "show ip cef" commands, which one does the router actually reference?

Also, how does this affect ACL processing? If the packets are being fast-switched would each packet still be matched against every ACL entry?

Thanks,

Rajesh

[ahvn]

Hi Rajesh,

In my understanding its the cef that takes precedence to route-cache.Its a normal practice to not use enable route-cache while cef is in place. I feel both are contradictory.

I don't think ACL will be affected with cef.The packets will surely be matched against ACL even if cef is in place.

Rgds,

Homin

[ruwhite]

This is correct. The router will try to cef switch packets first, and if they can't be cef switched, punt them to the next slower path, which would be fast switching, if that's enabled. If they can't be fast switched, they would be punted to the process switching path. A couple of notes on this, however. First, fast switching is gone from the code in some recent relase or another, though I don't remember where. Second, some hardware platforms (hardware based switching paths) will not punt packets to the next slower path, such as the Cisco 12000.

Russ.W

[rajesh444]

Russ,

Thanks. I noticed that when attempting to enable CEF on interface using "ip route-cache cef" route-cache [Fast-switching] also gets enabled. Is there any way to turn it off without turning CEF off?

Thanks,

Rajesh

[ruwhite]

Not that I know of... If you turn on cef, you still get fast. You can turn off distributed fast (really distributed optimum), I think, but I don't have a 7500 to check that on at the moment.

Fast switching is still used for switching multicast, so I should have said above: "fast switching is gone for _unicast_ traffic switching" above, rather than it's just gone. :-)

Russ.W