BigIP LTM Standby device is showing offline on the active device

I had the same issue when upgrading an HA pair from 11.5.1 to 12.1. After upgrading the standby was seen as offline.

I was on the phone for 4 hours. The device group was rebuilt, and still nothing.

What fixed it was support adding the devices management interface to the network failover interface list.

system-high avalibility-device connectivity-network failover

I still don't understand why, and the explanation made no sense, but it was now 6am and it was working...

The simple fact was I upgraded from 11.5.1 to 12.1 on a EXTREMELY stable HA pair that had a directly connected dedicated connection between the two units on interface 1.8 for the HA failover heartbeat in addition to the serial HA cable, and the HA pairing broke.

And the solution was to rebuild the trust group, and add the management interface...and there was the left over original trust group visable in the GUI that I never got an explination why it is still there, but again it was 6am...but interesting that group has a value of hidden in it.

So from my experience...if you are going to upgrade from to 12.x, add you management ips to the network failover ip list before you upgrade in addition to whether you use the hardware cable, or a dedicated ethernet cross connect...

cm device-group /Common/device_trust_group { auto-sync enabled devices { /Common/xxxltm3.xxx.com { } /Common/xxxltm4.xxx.com { } } hidden true network-failover disabled }