L3 ECMP over links with different RTT

Iljitsch van Beijnum iljitsch at muada.com
Tue May 10 14:37:08 UTC 2011

On 10 mei 2011, at 16:28, Dikkema, Michael (Business Technology) wrote:

> Is it foolish to build a L3 ECMP connection between 2 iBGP routers with one of the links having a 50% longer RTT?

No problem at all as long as you don't do per-packet load balancing but something that makes sure a single flow only goes over a single link. There are many ways to skin that particular cat, best practice is based on the 5-tuple (source and dest addresses and ports and the protocol number) which will give you the best chance of having a similar load on both links as long as you have at least some 1000 flows at any given time. With less granular load balancing there's a much bigger risk that one link will be full and the other more or less idle unless you have very, very many flows. You may want to use VLANs so you can load balance some stuff as per the above and manually balance some other stuff to go over the faster link.

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