The below post by Waylon Horne:

Quick Steps to F5 BIG-IP LTM 11.x Virtual Edition High Availability

Acting as a reverse proxy and distributing network and application traffic amongst numerous servers is just the tip of the iceberg when it comes to the F5’s many features.
With today’s increasing demand for capacity, the F5 is becoming one of the most crucial devices in the networking jungle.

And with this demand, it’s only logical that it should be configured to be always available, hence the quick steps below to help you configure your F5 for HA.

1: Prerequisites:

1. You must have a dedicated VLAN for HA.
2. You must have Self-IP’s configured for both the HA VLAN network and Management network on both devices.
3. These steps assume that you have setup the necessary defaults such as DNS, NTP, Admin Password, VLANS, etc.

2: HA VLAN setup:

1. Login at https://YourLTMApplianceIP Enter the admin username/password.
2. Go to Network > VLAN > VLAN List. Click
3. Enter name
4. Select 1.3 for interface, Tagging Click the Add button.
5. Click
6. Repeat the same steps as above on Appliance B.

3: Self IP setup:

1. Go to Network > Self IPs. Click Create
2. Type Name as SIP-HighAvailability. IP Address 0.0.1. Netmask as 255.255.255.0. VLAN as HighAvailability. Port lockdown Allow All. Select the Default Traffic Group (non-floating).
3. Repeat the same steps as above on Appliance B using 0.0.2 as the IP.
4. Click Finished.

4: Getting ready for HA setup:

1. On Appliance A go to Device Management > Devices > Device Connectivity > Config Sync. Select the0.0.1(HighAvailability) VLAN IP.
2. On Appliance A go to Device Management > Devices > Device Connectivity > Failover Network. Click Addunder Failover Unicast Configuration. Select your Management IP here.
3. Repeat the same steps as above on Appliance B using Appliance B’s HA and Management IP.

5: HA cluster setup:

1. On Appliance A go to Device Management > Device Trust > Peer List. Click Use the Management IP of Appliance B and admin username/password. Follow the rest of the steps
2. Both appliances should now be paired with each other.
3. On Appliance A go to Device Management > Device Groups. Click Create.
4. Type name as SyncFailover to identify the device group which will participate in failover cluster.
5. Group Type is Sync-Failover.
6. Drag both IPs from right to left.
7. Select Full Sync and Network Failover (leave Automatic Sync Unchecked).
8. On Appliance A go to Device Management > Overview and select SyncFailover under Device Groups
9. Select Appliance A under Devices, then select Sync Device to Group as well as the ticking the Overwrite Configuration check box. Click Sync.
10. You HA cluster Setup is now complete. One appliance will now show as ACTIVE and the other one STANDBY.

More HA troubleshooting tips can be found on F5’s support KB article “SOL13946: Troubleshooting ConfigSync and device service clustering issues (11.x)”