Hi,

In case the cable fails :

The primary unit MAC address is always coupled with the active IP addresses. The exception to this rule occurs when the secondary unit is active and cannot obtain the primary MAC address over the failover link. In this case, the secondary MAC address is used.

If the secondary unit boots and does not detect the primary unit, it becomes the active unit. It uses its own MAC addresses for the active IP addresses. When the primary unit becomes available, the secondary unit changes the MAC addresses to those of the primary unit, which can cause an interruption in your network traffic. In order to avoid this, configure the failover pair with virtual MAC addresses.

Plz refer to this link for details :

http://www.cisco.com/en/US/products/hw/vpndevc/ps2030/products_configuration_example09186a00807dac5f.shtml

Raj