Modern firewalls, load balancers, and most stateful devices support the concept of virtual context, which is the ability to support multiple virtual firewalls or virtual load balancers. Up to 250 virtual contexts, fully autonomous and isolated from each other, can be enabled on a single physical appliance or service module.
To offer the high availability service required for business continuity, firewalls and load balancers work by pairing physical devices. Both devices remain active while the virtual contexts run in an active/standby fashion. In addition to providing redundancy, this mechanism distributes the active context between the two devices, improving the total throughput for active workflows.
When interconnecting multiple data centers and deploying firewalls and other stateful devices such as load balancers, the distance between remote sites is an important consideration.
When data center sites are deployed in close proximity (such as within the few kilometers that is typical for large campus deployments), they can be considered a single, physically-stretched data center location. Under these premises, it would probably be acceptable to deploy the stateful devices in a stretched fashion, with one physical member of the HA pair in each data center site. For deployments where the distance between locations is farther, a pair of HA devices is typically deployed in each physical data center.
There are some important reasons to maintain each active/standby pair within the same data center. Redundancy is controlled by two devices, which means that dispersing the active/standby contexts in two different locations would limit the maximum number of data centers to two. On the other hand, keeping the active/standby pair of network services inside the same physical data center, allows replicating the same security policies in more than two data centers.
In addition, the link between the physical devices used for health check and process synchronization (replication of the active flows for stateful failover) must be extended in a very solid fashion. Due to its function of fault tolerance, it is also very sensitive to latency.
Last but not least, security and optimization functions usually require maintaining a stateful session. Therefore, for the same session, the traffic should be returned to the original virtual context that acknowledged the first flow, otherwise the flow will be dropped.
This behavior of symmetrical paths should be controlled and maintained, especially with the migration of VMs over a LAN extension as explained in the next topics.