additional Trunk link between 2 LTM load balancer in HA vlan

aren't you also using management interface for network failover?

could forcing unit offline be useful?

sol15122: Overview of the Force Offline option

NO, mgmt interface not using for failover. instead we are using HA vlan with a trunk link between two LTM load balancer as failover link......

ok for force offline ...... what you suggest about network failover option in high availability section in LTM

NO, mgmt interface not using for failover.

it is the best practice (to also setup network failover via management interface).

F5 recommends that you configure network failover to communicate over the management network, in addition to a TMM network. Configuring network failover in this way creates a reliable failover communication channel while the system initializes, and before the TMM network connectivity channels are fully established. To do this you must configure a failover unicast address and a failover multicast address for each device group member.

sol14135: Defining network resources for BIG-IP high-availability features (11.x)

Yes. Management link pretty much MUST be used for failover. Because its the only link to come up in time that a rebooting unit wont take over the service before it realises it doesnt need to...