Posted on July 18, 2018 5:26 pm
 |  Asked by Desislava Dimitrova
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I need to determine the ECMP path (route) to be taken by a specific packet (combination of packet header fields). I use an Arista 7050TX switch with EOS v. 4.18.6M . It seems there is a command in config mode (show load-balance destination ingress-interface {used header fields}) which helps determine the outgoing interface for a specific packet. The Arista blog post however is not publicly visible. Could someone provide a detailed explanation of the command?

Alternatively and preferably, I would like to reproduce the hashing algorithm behaviour. My attempts to track that so far bare no results. I can set the seed, offset and header fields to use but I need to know: the polynomial used in the CRC calculation; if in or out CRC inversion is used; and how is the mapping of hash ranges to ECMP buckets defined. Is this information available and could someone point me in the right direction?

Answered on July 18, 2018 7:39 pm

As you mentioned, the command output provides visibility into ECMP hash i.e., it gives you the output interface from an egress ECMP set based on the flow parameters supplied by the user.

Here’s an example:

7050#show load-balance destination ingress-interface Ethernet1/1 src-ipv4-address dst-ipv4-address ip-protocol 17 src-l4-port 11001 dst-l4-port 19872
Output Interface: Port-Channel102

I presume you meant the polynomial used for the hash function. The specific polynomials used for ECMP/LAG hash algorithms is abstracted to the user and not exposed outside.

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