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Citrix SD-WAN Platforms
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How High-Availability Mode Works

May 23, 2019
Contributed by: 
C

In a high availability (high availability) pair, one appliance is primary, and the other is secondary. The primary monitors its own and the secondary’s status. If it detects a problem, traffic processing fails over to the secondary appliance. Existing TCP connections are terminated. To ensure successful failover, the two appliances keep their configurations synchronized. In a WCCP mode high availability configuration, the appliance that is processing traffic maintains communication with the upstream router.

Status monitoring—When high availability is enabled, the primary appliance uses the VRRP protocol to send a heartbeat signal to the secondary appliance once per second. In addition, the primary appliance monitors the carrier status of its Ethernet ports. The loss of carrier on a previously active port implies a loss of connectivity.

Failover If the heartbeat signal of the primary appliance should fail, or if the primary appliance loses carrier for five seconds on any previously active Ethernet port, the secondary appliance takes over, becoming the primary. When the failed appliance restarts, it becomes the secondary. The new primary announces itself on the network with an ARP broadcast. MAC spoofing is not used. Ethernet bridging is disabled on the secondary appliance, leaving the primary appliance as the only path for inline traffic. Fail-to-wire is inhibited on both appliances to prevent loops.

Warning

The Ethernet bypass function is disabled in high availability mode. If both appliances in an inline high availability pair lose power, connectivity is lost. If WAN connectivity is needed during power outages, at least one appliance must be attached to a backup power source. Note

The secondary appliance in the high availability pair has one of its bridge ports, port apA.1, disabled to prevent forwarding loops. If the appliance has dual bridges, apB.1 is also disabled. In a one-arm installation, use port apA.2. Otherwise, the secondary appliance becomes inaccessible when high availability is enabled.

Primary/secondary assignment—If both appliances are restarted, the first one to fully initialize itself becomes the primary. That is, the appliances have no assigned roles, and the first one to become available takes over as the primary. The appliance with the highest IP address on the interface used for the VRRP heartbeat is used as a tie-breaker if both become available at the same time.

Connection termination during failover—Both accelerated and unaccelerated TCP connections are terminated as a side effect of failover. Non-TCP sessions are not affected, except for the delay caused by the brief period (several seconds) between the failure of the primary appliance and the failover to the secondary appliance. Users experience the closing of open connections, but they can open new connections.

Configuration synchronization—The two appliances synchronize their settings to ensure that the secondary is ready to take over for the primary. If the configuration of the pair is changed through the browser based interface, the primary appliance updates the secondary appliance immediately.

high availability cannot be enabled unless both appliances are running the same software release.

high availability in WCCP mode—When WCCP is used with an high availability pair, the primary appliance establishes communication with the router. The appliance uses its management IP address on apA or apB, not its virtual IP address, to communicate with the router. Upon failover, the new primary appliance establishes WCCP communication with the router.

How High-Availability Mode Works
May 23, 2019
Contributed by: 
C
May 23, 2019
Contributed by: 
C

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