Commands Usage

Unanswered Question
Dec 17th, 2007

I have been noticing on most of our production routers, which I inherited from a forder engineer, that the following commands are being used:

- no ip route-cache cef

- no ip route-cache

- no fair-queue

Could someone please tell me why are those commands being negated? What's the idea behind disabling them?

Thanks in advance,

sK

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Richard Burts Mon, 12/17/2007 - 10:42

SK

the no ip route-cache cef is disabling CEF on an interface. The no ip route-cache is disabling fast switching on an interface. The result is to force process switching of packets on that interface. The reasons why this is being done are difficult to identify since it might be done for several reasons and we do not have enough information to know which one it might be. In some old versions of IOS (when CEF was a new technology) people would turn off CEF to avoid some bug that might be associated with CEF. There is little reason to do this with recent IOS versions. I have seen people turn off fast switching when they want the router CPU to process every packet when they were running debug. I guess that some people might turn off CEF and/or fast switching in an attempt to conserve memory in the router and not use memory in the CEF or fast switching tables.

Forcing packets to process switch will certainly impact performance of the router (it will require more CPU cycles than either fast switching or CEF switching) and it is difficult for me to think of a good reason to do this on other than a temporary basis.

no fair-queue will disable fair queue, which is a queuing method that is enabled by default on relatively low speed serial interfaces. The impact of reverting to FIFO queue instead of fair queue is more difficult to guage.

HTH

Rick

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