• If sessions are lost after a failover you may need to change route-ttl to keep synchronized routes active longer. See “Change how long routes stay in a cluster unit routing table”.

• To control which cluster unit becomes the primary unit, you can change the device priority and enable override. See “Controlling primary unit selection using device priority and override”.

• Changes made to a cluster can be lost if override is enabled. See “Configuration changes can be lost if override is enabled”.

• In some cases, age differences among cluster units result in the wrong cluster unit becoming the primary unit. For example, if a cluster unit set to a high priority reboots, that unit will have a lower age than other cluster units. You can resolve this problem by resetting the age of one or more cluster units. See “Resetting the age of all cluster units”. You can also adjust how sensitive the cluster is to age differences. This can be useful if large age differences cause problems. See “Cluster age difference margin (grace period)” and “Changing the cluster age difference margin”

• If one of the cluster units needs to be serviced or removed from the cluster for other reasons, you can do so without affecting the operation of the cluster. See “Disconnecting a cluster unit from a cluster”.

• The web‑based manager and CLI will not allow you to configure HA if:

• You have configured a FortiGate interface to get its IP address using DHCP or PPPoE. See “FortiGate HA compatibility with PPPoE and DHCP”.

• You have enabled VRRP. See “VRRP”.

• You have enabled TCP session synchronization. See “FortiGate Session Life Support Protocol (FGSP)”.

• Some third-party network equipment may prevent HA heartbeat communication, resulting in a failure of the cluster or the creation of a split brain scenario. For example, some switches use packets with the same Ethertype as HA heartbeat packets use for internal functions and when used for HA heartbeat communication the switch generates CRC errors and the packets are not forwarded. See “Heartbeat packet Ethertypes”.

• Very busy clusters may not be able to send HA heartbeat packets quickly enough, also resulting in a split brain scenario. You may be able to resolve this problem by modifying HA heartbeat timing. See “Modifying heartbeat timing”.

• Very busy clusters may suffer performance reductions if session pickup is enabled. If possible you can disable this feature to improve performance. If you require session pickup for your cluster, several options are available for improving session pickup performance. See “Improving session synchronization performance”.

• If it takes longer than expected for a cluster to failover you can try changing how the primary unit sends gratuitous ARP packets. See “Changing how the primary unit sends gratuitous ARP packets after a failover”.

• You can also improve failover times by configuring the cluster for subsecond failover. See “Subsecond failover” and “Failover performance”.

• When you first put a FortiGate unit in HA mode you may loose connectivity to the unit. This occurs because HA changes the MAC addresses of all FortiGate unit interfaces, including the one that you are connecting to. The cluster MAC addresses also change if you change the some HA settings such as the cluster group ID. The connection will be restored in a short time as your network and PC updates to the new MAC address. To reconnect sooner, you can update the ARP table of your management PC by deleting the ARP table entry for the FortiGate unit (or just deleting all arp table entries). You may be able to delete the arp table of your management PC from a command prompt using a command similar to arp -d.

• Since HA changes all cluster unit MAC addresses, if your network uses MAC address filtering you may have to make configuration changes to account for the HA MAC addresses.

• A network may experience packet loss when two FortiGate HA clusters have been deployed in the same broadcast domain. Deploying two HA clusters in the same broadcast domain can result in packet loss because of MAC address conflicts. The packet loss can be diagnosed by pinging from one cluster to the other or by pinging both of the clusters from a device within the broadcast domain. You can resolve the MAC address conflict by changing the HA Group ID configuration of the two clusters. The HA Group ID is sometimes also called the Cluster ID. See “Diagnosing packet loss with two FortiGate HA clusters in the same broadcast domain”.

• The cluster CLI displays slave is not in sync messages if there is a synchronization problem between the primary unit and one or more subordinate units. See “How to diagnose HA out of sync messages”.

• If you have configured dynamic routing and the new primary unit takes too long to update its routing table after a failover you can configure graceful restart and also optimize how routing updates are synchronized. See “Configuring graceful restart for dynamic routing failover” and “Controlling how the FGCP synchronizes kernel routing table updates”.

• Some switches may not be able to detect that the primary unit has become a subordinate unit and will keep sending packets to the former primary unit. This can occur after a link failover if the switch does not detect the failure and does not clear its MAC forwarding table. See “Updating MAC forwarding tables when a link failover occurs”.

• If a link not directly connected to a cluster unit (for example, between a switch connected to a cluster interface and the network) fails you can enable remote link failover to maintain communication. See “Remote link failover”.

• If you find that some cluster units are not running the same firmware build you can reinstall the correct firmware build on the cluster to upgrade all cluster units to the same firmware build. See “Synchronizing the firmware build running on a new cluster unit”.