Failover on Cisco ASA

Configuring high availability 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 if specific failover conditions are met. If those conditions are met, failover occurs.

The ASA supports two failover configurations, Active/Active failover and Active/Standby failover. Each failover configuration has its own method for determining and performing failover.

With Active/Active failover, both units can pass network traffic. This also lets you configure traffic sharing on your network. Active/Active failover is available only on units running in multiple context mode.

With Active/Standby failover, only one unit passes traffic while the other unit waits in a standby state. Active/Standby failover is available on units running in either single or multiple context mode.

Both failover configurations support stateful or stateless (regular) failover.

Hardware Requirements

The two units in a failover configuration must be the same model, have the same number and types of interfaces, and the same SSMs installed (if any).

If you are using units with different Flash memory sizes in your failover configuration, make sure the unit with the smaller Flash memory has enough space to accommodate the software image files and the configuration files. If it does not, configuration synchronization from the unit with the larger Flash memory to the unit with the smaller Flash memory will fail.

Although it is not required, it is recommended that both units have the same amount of RAM memory installed.

Software Requirements

The two units in a failover configuration must be in the same operating modes (routed or transparent, single or multiple context). They must have the same major (first number) and minor (second number) software version. However, you can use different versions of the software during an upgrade process; for example, you can upgrade one unit from Version 7.0(1) to Version 7.0(2) and have failover remain active.

The two units in a failover pair constantly communicate over a failover link to determine the operating status of each unit. The following information is communicated over the failover link:

•The unit state (active or standby)

•Hello messages (keep-alives)

•Network link status

•MAC address exchange

•Configuration replication and synchronization

All information sent over the failover and Stateful Failover links is sent in clear text unless you secure the communication with a failover key. If the ASA is used to terminate VPN tunnels, this information includes any usernames, passwords and preshared keys used for establishing the tunnels. Transmitting this sensitive data in clear text could pose a
significant security risk. We recommend securing the failover communication with a failover key if you are using the ASA to terminate VPN tunnels.

You can use any unused Ethernet interface on the device as the failover link; however, you cannot specify an interface that is currently configured with a name. The LAN failover link interface is not configured as a normal networking interface; it exists for failover communication only. This interface should only be used for the LAN failover link (and optionally for the Stateful Failover link).

Connect the LAN failover link in one of the following two ways:

•Using a switch, with no other device on the same network segment (broadcast domain or VLAN) as the LAN failover interfaces of the ASA.

•Using a crossover Ethernet cable to connect the appliances directly, without the need for an external switch.

When you use a crossover cable for the LAN failover link, if the LAN interface fails, the link is brought down on both peers. This condition may hamper troubleshooting efforts because you cannot easily determine which interface failed and caused the link to come down.

The ASA supports Auto-MDI/MDIX on its copper Ethernet ports, so you can either use a crossover cable or a straight-through cable. If you use a straight-through cable, the interface automatically detects the cable and swaps one of the transmit/receive pairs to MDIX.