How does CSS ecmp address with prefer-ingress works?

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Aug 25th, 2007
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We have four isp routers and one CSS connected to a layer 2 switch. The CSS is configured with four equal cost default routes to the routers. Since all incoming traffic is entering from the same interface of the CSS, how does the CSS know which routers to send the reply packets to?

We are experiencing problems with return packets not being sent back to the right

router. However if we reduce the isp routers to 3 from 4, things seems to work. We were told that we just got lucky with the ecmp hashing algorithm but

we could not convince others in our organization that was the case. Can you share some insights how the ecmp hashing algorithm works. And why 3 routers works better than 4 routers in our topology.

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Syed Iftekhar Ahmed Mon, 08/27/2007 - 15:09
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This decision ismade on the basis of client's flow.


The algo to decide the next hop is as follows


1. CSS XORs the Src & Dest IP

2. CSS then MOD the result with number of equal cost default routes and gets the index.

3. It decides on the next hop based on this index.


you can put your values in the above algo and you will be able to get the answer.


Syed iftekhar Ahmed

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