ASR9000 ECMP LOAD BALANCING P ROUTERS

[Amauri Soto Rodriguez]

Hi Everyone.

Currently I'm facing a polarization issue in some ASR9010 devices with RSP440 and Typhoon cards, those devices have a P role on the network, with even number links connected to balance in L3 using a OSPF the scenario as described below

                        +++++++++++                    +++++++++    MPLS    +++++++++     

                        +PW no Cisco+-------------  +  c7600    +----4xGi----+  ASR9K  +

                         +++++++++++                   +++++++++    OSPF    +++++++++

According with the ECMP algorithm used by the typhoon cards the balance is done based on traffic flows, and it is computed using the inner most label when there are more than 3 labels and the router id by a CRC operations which considers up to 32 combinations (5 bits).

In my investigation I´ve seen the router is balancing the flows with not so much difference among the outgoing interfaces 4 flows for Gi0/3/0/0 , 6 flows for Gi0/3/0/1, 3 flows for Gi0/3/0/2, 5 flows for Gi0/3/0/5.

To modify the behavior the load balancing schema I've changed the number of bits which the HASH algorithm uses, I tried with the 32 combinations, but the results were not different I could not get an homogenous load balancing schema.

My question are:

Has someone tried a different WA to get a more even traffic over the parallel links?

How can I know the bit string that router uses for the hash algorithm to predict which path the flow will take?

I appreciate your thoughts

[Aleksandar Vidakovic]

With such a small number of flows you can not expect to have a perfect flow distribution because the output interface selection ultimately depends on the innermost MPLS label value. You would need perfect distribution of MPLS label values to achieve near perfect distribution.

Have you considered deploying FAT PW?

[Amauri Soto Rodriguez]

Hi Aleksandar thanks for the reply.

Regarding your comment I think FAT can only be implemented on the PE routers which have the PW connections, I can suggest the customer to implement it on his devices since those are different cisco devices. Regarding the P router, the only way to try to correct the flow polarization will be changing the hash values?

Thanks.

[Aleksandar Vidakovic]

That is correct, if the PE routers don't support FAT PW, the only thing you can do on the P router is to move the hash.

regards,

/Aleksandar

[Amauri Soto Rodriguez]

Thanks for your answers Aleksandar.

Regards.