Fault F1370

After working for ~8 years with Cisco ACI, I faced a new fault this morning at one of my customers.

This sounded strange, node-4 is the Standby APIC in this Fabric, so it should just sit idle and do nothing.

Both messages pointed to more or less the same, that this Standby APIC suddenly started to communicate with the APIC Cluster.

Both messages have the same ACI Fault Code F1370, but are slightly different.

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Messages from a controller (node-4) with the Globally Unique Identifier (GUID)
X_X and the IP address 1.1.1.1 started arriving at X and are being rejected
because: Expansion contender is standby

Messages from a controller (node-4) with the Globally Unique Identifier (GUID)
X_X and the IP address 1.1.1.1 started arriving at X and are being rejected
because: Source is not commissioned

How to clear the Fault F1370

As this was the first time I saw this issue, I decided to go for the always working fix in IT… just reboot the Standby APIC controller. This cleared the fault, but I will further investigate if I can find more information about the issue.