Summary
After replacing a faulty switch in the VAST cluster and applying the configuration, the two MLAG ports (going to the external network) would not come up on the switch, though the ports on the partner switch were still functioning.
Looking at the show mlag output, we could see that there was a MAC address mismatch:
vast-SW01 [switch1: master] (config) # show mlag
Admin status: Enabled
Operational status: Up
Reload-delay: 30 sec
Keepalive-interval: 1 sec
Upgrade-timeout: 60 min
System-mac: C4:65:EC:62:B0:DD [Mismatched] <---------
MLAG Ports Configuration Summary:
Configured: 1
Disabled: 0
Enabled: 1
MLAG Ports Status Summary:
Inactive: 0
Active-partial: 0
Active-full: 1
MLAG IPLs Summary:
-----------------------------------------------------------------------------------------------------------------------------------------------------------------
ID Group Vlan Operational Local Peer Up Time Toggle Counter
Port-Channel Interface State IP address IP address
-----------------------------------------------------------------------------------------------------------------------------------------------------------------
1 Po1 4000 Up 10.10.10.2 10.10.10.1 0 days, 00:16:39 5
MLAG Members Summary:
---------------------------------------------------------------------
System-id State Hostname
---------------------------------------------------------------------
1C:34:DA:A8:C5:08 Up <vast-SW01>
98:03:9B:FF:45:C8 Peering vast-SW02
Further, the ports in the MLAG, ethernet 1/13 and ethernet 1/14, were down (suspended)
Solution
In order to resolve this, set the MLAG MAC address on the replaced switch to match the MLAG partner switch with the command mlag system-mac <MAC address>