Mac table, STP blocked port and ubiquiti discovery

[aurelien personnaz]

Hello,

I have noticed a surprising behavior concerning a STP blocked port and the ubiquiti discovery.

I have two switches, let's call them A and B.

Both are (approximately) connected to the same uplink and connected together.
On B, STP blocks the link to A.

On A, the uplink is on the port 1 and the link to B is on the port 2 (STP blocked).
Since the link from A to B is blocked, B is supposed to appear on the port 1 in the mac table of A, and it does most of the time.

But here is the surprising thing. Every time I run a ubnt discovery, B appears on the port 2 in the mac table of A.
This behavior does only concern A, as A does always appear on the uplink port in the mac table of B.

That does probably mean that the ubnt discovery multicast packets are passing through the STP blocked port on B...
It is also surprising that A knows the mac of B on a given link and B knows the mac of A on another.

I don't know whether it's a bug or not, but it does seem pretty strange so we thought it would be worth sharing with you.
But overall it does not seem to have any negative effect...

[Eric Stern]

I can't duplicate this in the lab. If you can provide detailed configuration and steps to reproduce it I'll look into it further.