04-19-2015 03:13 AM - edited 03-05-2019 01:17 AM
Hi,
What is the concept behind ECMP (equal cost multi path) ? Is it different for EIGRP , OSPF , ISIS etc ?
thanks
Solved! Go to Solution.
04-19-2015 05:56 AM
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
04-19-2015 05:56 AM
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
04-20-2015 07:17 AM
Disclaimer
The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.
Liability Disclaimer
In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.
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.
Discover and save your favorite ideas. Come back to expert answers, step-by-step guides, recent topics, and more.
New here? Get started with these tips. How to use Community New member guide