[c-nsp] Help with MAC control on redundant L2 links.

Howard Leadmon howard at leadmon.net
Sat Nov 12 16:53:44 EST 2022

  I have an issue I am trying to sort out that I haven't run into 
before, and figured someone might be able to give me a few pointers.

  I have three sites lets say A, B and C, and they all have redundant 
connectivity via two Layer 2 fiber loops with two different carriers.

  We use Comcast and Zayo to reach the various sites, but realized that 
I was having connectivity issues, but after talking with Comcast, they 
are informing me the issue is the MAC being presented from different 
locations at the same time.

So say at Site-A I am presenting a mac ending in 1701, I of course 
present this to both Comcast and Zayo, as expected.   Now at Site-B, I 
am being informed that when my switch receives that 1701 down the loop 
from Zayo, it is of course presenting it back to Comcast as a valid 
MAC.  As such they say they are learning this MAC from multiple 
locations at the same time, and they can only support learning it from 
one point, so they drop the MAC.   Of course Site-C has the same issue, 
also presenting what it knows from the other points.

I thought setting 'spanning-tree link-type shared' allowed it to handle 
this, but I am guessing not well enough.  Well it might let the Cisco 
handle it, but apparently is playing havoc with the Ciena switches that 
Comcast is using.

I looked at setting a mac filter (maybe I am looking at this wrong) to 
say if you saw this coming in, don't resend it back out to any other 
place. The issue I saw, was it only allowed it to be an ingress filter, 
which means I would discard the address completely which doesn't seem 
good either.

I am sure there is a right way to handle this, but honestly not 
something I have encountered before.  If anyone could give me any hints, 
or point me to something that might help it would be appreciated..

Howard Leadmon - howard at leadmon.net
PBW Communications, LLC

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