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Port down in Link aggregation triggers a TCN

Johan de Greef
Occasional Contributor

Port down in Link aggregation triggers a TCN

Hi All,

In our network we run multiple different types of HP switches. We also run 2 HP A7506-V switches in a dual datacenter setup. We have 2 darkfiber connections running between the 2 datacenters and both are connected to the HP A7506-V switches. These 2 links are configured to form a link-aggregation. We recently had an incident where one of the darkfiber connections went down for a brief period of time. When one of the interfaces in the BAGG went down a TCN was generated on one of the switch and some of the ports went into the default discarding state of 30 seconds due to this TCN.

We are running MST with 2 instances on top of instance 0 (so 3 in total). The TCN was generated in instance 0. The ports which went into discarding state are in a VLAN which is assigned to instance 0.

I checked the STP status of the interfaces and the STP status on both links is CIST disabled port. The BAGG is the CIST root port. I appears that STP is only monitoring the BAGG, what I expected. I thought that when a BAGG is configured STP monitors the BAGG and not the individual links inside the BAGG. So in this case a TCN should not be generated because the BAGG itself stayed up (the other link was still functioning).
My question is this, why was the TCN generated? 

Thanks in advance!