I was in a training class recently and they were speaking about ECMP and how it “converges” if a link goes down. Let me just say this, that is absolutely incorrect and is just as bad as saying “I have two class C’s”, it really doesn’t bode well with most people.

With ECMP you’re actually installing multiple routes of the same cost into the routing table and you’re either going to load balance based on a per-packet or per flow basis with per-flow being the most preferred because of the nature of TCP operations. Now, how it load balances on which link will be determined upon the algorithm used, most use round-robin.

Please understand, ECMP doesn’t mean the links are of EQUAL bandwidth and latency, just from a metric cost perspective they’re “equal”. When a link goes down there is absolutely no convergence taking place, the packets/flow just get routed out of one of the other available, equal-cost links. Please stop saying they’re “converging” because that makes most think there is either a dynamic computation taking place with a dynamic routing protocol or the router itself is having to install a route into the RIB from the FIB.


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