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Cisco ASA 5500 Series Configuration Guide using ASDM
Chapter68 Configuring IKE, Load Balancing, and NAC
Configuring Load Balancing
•Configuring the participating device by enabling load balancing on the device and defining
device-specific properties. These values vary from device to device.
Creating Virtual Clusters
To implement load balancing, you group together logically two or more devices on the same private
LAN-to-LAN network into a virtual cluster.
All devices in the virtual cluster carry session loads. One device in the virtual cluster, the virtual cluster
master, directs incoming calls to the other devices, called backup devices. The virtual cluster master
monitors all devices in the cluster, keeps track of how busy each is, and distributes the session load
accordingly. The role of virtual cluster master is not tied to a physical device; it can shift among devices.
For example, if the current virtual cluster master fails, one of the backup devices in the cluster takes over
that role and immediately becomes the new virtual cluster master.
The virtual cluster appears to outside clients as a single virtual cluster IP address. This IP address is not
tied to a specific physical device. It belongs to the current virtual cluster master; hence, it is virtual. A
VPN client attempting to establish a connection connects first to this virtual cluster IP address. The
virtual cluster master then sends back to the client the public IP address of the least-loaded available host
in the cluster. In a second transaction (transparent to the user) the client connects directly to that host.
In this way, the virtual cluster master directs traffic evenly and efficiently across resources.
Note All clients other than the Cisco VPN client, the Cisco VPN 3002 Hardware Client, or the ASA 5505
operating as an Easy VPN Client 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. Should the virtual cluster master itself fail, 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
A load-balancing cluster can consist of ASAs of the same release, of mixed releases, as well as VPN
3000 concentrators, or a mixture of these, subject to the following restrictions:
•Load-balancing clusters that consist of both same release ASAs and VPN 3000 concentrators can
run load balancing for a mixture of IPsec, AnyConnect, and clientless SSL VPN client and clientless
sessions.
•Load-balancing clusters that include mixed release ASAs or same release ASAs and VPN 3000
concentrators or both can support only IPsec sessions. In such a configuration, however, the ASAs
might not reach their full IPsec capacity. “Scenario 1: Mixed Cluster with No SSL VPN
Connections” on page 25, illustrates this situation.
Since Release 7.1(1), IPsec and SSL VPN sessions count or weigh equally in determining the load that
each device in the cluster carries. This represents a departure from the load balancing calculation for the
ASA Release 7.0(x) software and the VPN 3000 concentrator, in that these platforms both use a
weighting algorithm that, on some hardware platforms, calculates SSL VPN session load differently
from IPsec session load.
The virtual master of the cluster assigns session requests to the members of the cluster. The ASA regards
all sessions, SSL VPN or IPsec, as equal and assigns them accordingly. You can configure the number
of IPsec and SSL VPN sessions to allow, up to the maximum allowed by your configuration and license.