Cisco ASA Series Cli Configuration Manual page 1620

Understanding Load Balancing
Note
All clients other than the Cisco VPN client or the Cisco 3002 hardware client should connect directly to
the ASA as usual; they do not use the virtual cluster IP address.
If a machine in the cluster fails, the terminated sessions can immediately reconnect to the virtual cluster
IP address. The virtual cluster master then directs these connections to another active device in the
cluster. If the virtual cluster master itself fails, a backup device in the cluster immediately and
automatically takes over as the new virtual session master. Even if several devices in the cluster fail,
users can continue to connect to the cluster as long as any one device in the cluster is up and available.
Comparing Load Balancing to Failover
Both load balancing and failover are high-availability features, but they function differently and have
different requirements. In some circumstances you can use both load balancing and failover. The
following sections describe the differences between these features.
Load Balancing
Load balancing is a mechanism for equitably distributing remote-access VPN traffic among the devices
in a virtual cluster. It is based on simple distribution of traffic without taking into account throughput or
other factors. A load-balancing cluster consists of two or more devices, one is the virtual master, and the
other devices are the backup. These devices do not need to be of the exact same type, or have identical
software versions or configurations.
All active devices in a virtual cluster carry session loads. Load balancing directs traffic to the
least-loaded device in the cluster, distributing the load among all devices. It makes efficient use of
system resources and provides increased performance and high availability.
Failover
A failover configuration requires two identical ASAs connected to each other through a dedicated
failover link and, optionally, a stateful failover link. The health of the active interfaces and units is
monitored to determine when specific failover conditions are met. If those conditions occur, failover
occurs. Failover supports both VPN and firewall configurations.
The ASA supports two failover configurations: Active/Active failover and Active/Standby failover.
With Active/Active failover, both units can pass network traffic. This is not true load balancing, although
it might appear to have the same effect. When failover occurs, the remaining active unit takes over
passing the combined traffic, based on the configured parameters. Therefore, when configuring
Active/Active failover, you must make sure that the combined traffic for both units is within the capacity
of each unit.
With Active/Standby failover, only one unit passes traffic, while the other unit waits in a standby state
and does not pass traffic. Active/Standby failover lets you use a second ASA to take over the functions
of a failed unit. When the active unit fails, it changes to the standby state, while the standby unit changes
to the active state. The unit that becomes active assumes the IP addresses (or, for transparent firewall,
the management IP address) and MAC addresses of the failed unit and begins passing traffic. The unit
that is now in standby state takes over the standby IP addresses of the active unit. If an active unit fails,
the standby takes over without any interruption to the client VPN tunnel.
Cisco ASA Series CLI Configuration Guide
1-8
Chapter 1 Setting General VPN Parameters