Before reloading an MLAG peer switch to perform an upgrade/downgrade, one of the key check-ups is to make sure the STP is restartable. Otherwise, traffic loss is expected.
bn303.08:17:13(s1)#sh spanning-tree instance detail | grep rest
Stp agent restartable : False
Oct 19 08:16:15 bn303.aristanetworks.com Stp: %SPANTREE-6-STABLE_CHANGE: Stp state is now not stable
Oct 19 08:17:06 bn303.aristanetworks.com Stp: %SPANTREE-6-STABLE_CHANGE: Stp state is now stable
To find out which ports causing this issue, by using CLI - sh spanning-tree topology status detail, to search most recently changed ports, like
bn303.08:22:51(s1)#sh spanning-tree topology status detail | more
Topology: Cist
Mapped Vlans: 1,4080-4087
Cpu: forwarding (1 changes, last 7:40:20 ago)
Ethernet8/16/1: forwarding (3 changes, last 0:23:10 ago) <<<
PeerEthernet8/16/1: forwarding (3 changes, last 0:23:10 ago) <<<
The reason of unstable STP is because, the MLAG switch keeps receiving its own (actually from the active MLAG peer) BPDU and can't form an agreement. The solution can be either, shutting down the port or put it as no-switching port.
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