ECMP - equal cost multi path

[Arjun Dabol]

Hi,

What is the concept behind ECMP  (equal cost multi path) ? Is it different for EIGRP , OSPF , ISIS etc ?

thanks

[Jon Marshall · Accepted solution]

While different routing protocols may have different limits as to how many equal cost paths can be used the principle is the same including static routes.

If a router has equal cost paths to a destination network then it will use all those paths to send traffic to the destination.

It can do this this in one of two ways -

1) per destination which means for a specific destination all traffic is sent over the same link

or

2) per packet which means each packet is sent over one of the links and then the next packet is sent over another of the links etc.

The default is per destination and is the one most commonly used.

Using per packet load balancing can result in packets being delivered to the destination out of order because each packet is using a different link and this can have performance implications for the receiving application.

On most modern devices CEF is used to do the actual forwarding of the traffic so I've included a link which goes into how it works in much more detail -

http://www.cisco.com/en/US/products/hw/modules/ps2033/prod_technical_reference09186a00800afeb7.html

Jon

[Joseph W. Doherty]

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Posting

The concept behind ECMP is to actively and concurrently take advantage of multiple link/path bandwidth.

Oh, and to just add to what Jon has already noted, ECMP usually doesn't track actual load.  So, "seeing" unequal path/link loading, especially short term, isn't unusual.

PS:

There's also unequal cost multi-path routing too - EIGRP supports that.