Policy-Based Routing
When using Individual interfaces, each threat defense interface maintains its own IP address and MAC address. One method of load balancing is Policy-Based Routing (PBR).
We recommend this method if you are already using PBR, and want to take advantage of your existing infrastructure.
PBR makes routing decisions based on a route map and ACL. You must manually divide traffic between all threat defenses in a cluster. Because PBR is static, it may not achieve the optimum load balancing result at all times. To achieve the best performance, we recommend that you configure the PBR policy so that forward and return packets of a connection are directed to the same threat defense. For example, if you have a Cisco router, redundancy can be achieved by using Cisco IOS PBR with Object Tracking. Cisco IOS Object Tracking monitors each threat defense using ICMP ping. PBR can then enable or disable route maps based on reachability of a particular threat defense. See the following URLs for more details:
http://www.cisco.com/en/US/products/ps6599/products_white_paper09186a00800a4409.shtml