Here the configuration I use for ASA failover 8.4+ based on my experience:

On the primary:

interface GigabitEthernet0/4 no shutdown failover failover lan unit primary failover lan interface FAILINK GigabitEthernet0/4 failover interface ip FAILINK 169.254.255.249 255.255.255.252 standby 169.254.255.250 failover key 222Th3Hak3Y222 failover link FAILINK GigabitEthernet0/4 prompt hostname state priority

On the secondary:

interface GigabitEthernet0/4 no shutdown failover lan unit secondary failover lan interface FAILINK GigabitEthernet0/4 failover interface ip FAILINK 169.254.255.249 255.255.255.252 standby 169.254.255.250 failover key 222Th3Hak3Y222 failover link FAILINK GigabitEthernet0/4 failover

To validate:

show failover

Notes:

• About the stateful link / failover link, you can:

  • use one interface for each
  • use a redundant interface
  • use an etherchannel but only one interface in the EtherChannel is used

• The IP 169.254.x.x are link local addresses not meant to be routed. Perfect for HA link.

• The prompt is mandatory as it allow you to quickly validate the state of the cluster.