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Multipathing in SPBM

Multipathing in SPBM

New Contributor III

Hello All!  Really hoping for some feedback here.  It's come to my attention that in an equal cost path situation, SPBM will use the ODD B-VID (4051) to forward traffic via one path and the EVEN B-VID (4052) to forward traffic on another.  I believe this is dependent on the i-sid's being EVEN vs ODD.  At any rate, we've noticed that our switches will always prefer to forward traffic using the "Blue" path in the picture attached even if two equal cost paths exist when the destination is that to an ODD v-IST BMAC.  

I tested this using l2 traceroutes, and although both switches in the v-IST cluster own the same virtual BMAC, it seems like the switch one of the switches owns it on 4051 and the other on 4052 but both respond.  My question is, is multipathing within SPBM on by default?  It doesn't seem this way because I have to manually lower the cost of the link on the RED path to force traffic to go over that path primarily (and it's only because its more cost effective to go there and then across the v-IST trunk).  Is there a way to force traffic to egress from the switch on VLAN 4052?

Also, can someone outline the use case for STP-Multi-Homing within SPBM and if it applies here?


New Contributor III

Hi Conft,

Don't know about others, but I can only see your attached picture in the smallish thread preview, and not here.
I think I remember Ed Koehler demonstrated in one of his YT videos detailed commands to list available paths in the fabric, and how I-SIDs /  VLANs got balanced the way you describe.
Maybe this video :

Without the picture, I'm not sure what's your architecture like, but AFAIK link aggregation based on vIST should only be used at the edge on the fabric.
And STP-multi-homing is basically STP support across the fabric, and should be IMHO be enabled by default. Not implementing STP on any network is just looking for trouble.

This is a tour of the link state database that is generated in each Fabric Connect node as they join the fabric domain. Our third step into the video series allows us to view the fabric domain and the services that reside within it.