F5 LTM hardware failover port issue

cable state 0 ?? is that correct behavior?

 show sys failover ?
Options:
  cable  Displays the status that the failover daemon detects on the serial
         cable from its failover peer. It also shows what the failover peer
         detects on the serial cable. An active BIG-IP will see a zero from its
         failover peer. A standby BIG-IP will see a one from its failover peer.

cable state 0 is normal behavior but if you unplug the cable you should see "cable state unset"

also what if my VLAN trunk failed in that case how does failover will work ?

You must enable vlan failsafe to detect vlans failures.