Tuesday, November 4, 2008

CEF LoadbalancingProblem

Sometimes you may face the problem in which if you ping from R7 to R6 it works perfect but when you try to ping from R5 to R6 you may face drops. Whats the reason for that ? Do you know ?
Needn’t worry about that I will let you know the reason behind this. Actually what happens when you ping from R7 the packets are process switched and then from R5 they are CEF switched because of cef switched by default it takes per destination based. So your packets always path either R5-R3 or R5-R1. Lets assume packets takes R5-R3 path and R5-R1 paths has some CRC errors etc. So in this case your packets will always follow R5-R3 path never R5-R1 and you find no more drops. But in latter case when you ping from R5 to R6, you always find drops because packets originated from R5 always process switched, so packets are always load balanced across both paths and you may drops becasue of this.
You can check with the help of " show ip cef exact-route " command which will tell you which path is being taken by the packets.

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