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Barracuda Load Balancer ADC

Understanding Barracuda Load Balancer ADC High Availability

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For high availability (HA), you can cluster two Barracuda Load Balancer ADCs as an active-passive pair. Only one of the appliances is active and processes traffic at any time, but the two systems continuously share almost all configuration settings and monitor each other's health. The passive (backup) appliance does not load balance or monitor the services or real servers. For example, in the web interface of the passive appliance, all of the services and real servers on the BASIC > Services page have red health indicators.

Failover Conditions

The active appliance in an HA setup handles all of the traffic until one of the following conditions is encountered:

  • The passive appliance detects that the active appliance is no longer responsive on the Management (MGMT) interface.
  • The active appliance detects that any of the monitored interfaces or links is down.
  • Memory usage on the active Barracuda Load Balancer ADC exceeds 75% of total system memory.
  • You manually execute a failover.
  • The active appliance encounters a hardware failure (including a power failure) or a failure in one of its critical software modules.
  • There is a data path crash on the active appliance.

If any of these conditions is encountered, the passive appliance becomes active, processes and load balances traffic for all of the services, and (if enabled) performs security validation. It also sends out a gratuitous address resolution protocol (GARP) every minute; the passive appliance does not  issue any address resolution protocols (ARPs).

The active appliance is determined by the Virtual Router Redundancy Protocol (VRRP) specification. You must configure the clustered appliances with the same Cluster Shared Secret  and  Cluster Group ID. If other systems on the same subnet are also using VRRP, the Cluster Group ID must be unique.


Before you can cluster two Barracuda Load Balancer ADCs, they must be:

  • The same model.
  • Activated and upgraded to the same firmware version.
  • Able to access all real servers.
  • On the same physical network segment.
  • Able to reach each other on the MGMT interface.

In addition, the active appliance must be fully configured. For the complete list of service and network configurations that you must complete, see Services and Step 5 - How to Configure Your Network and Services.


When setting up Barracuda Load Balancer ADCs for high availability, Barracuda recommends the following:

  • Do not configure services on the passive (backup) appliance.
  • To speed up recognition of a newly active Barracuda Load Balancer ADC, disable Spanning Tree Protocol on the ports of the switch where the MGMT ports of the two Barracuda Load Balancer ADCs are connected. If it is a Cisco switch, enable Spanning Tree PortFast on the ports connected to the MGMT ports of the Barracuda Load Balancer ADCs. When the Barracuda Load Balancer ADC becomes active it sends out a gratuitous ARP. It continues to send a gratuitous ARP every minute; the passive system does not issue any ARPs.


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