How the Cluster Fits into Your Network
The cluster consists of multiple firewalls acting as a single unit. To act as a cluster, the firewalls need the following infrastructure:
Isolated, high-speed backplane network for intra-cluster communication, known as the cluster control link.
Management access to each firewall for configuration and monitoring.
When you place the cluster in your network, the upstream and downstream routers need to be able to load-balance the data coming to and from the cluster using Spanned EtherChannels. Interfaces on multiple members of the cluster are grouped into a single EtherChannel; the EtherChannel performs load balancing between units.