Device and Link Redundancy Overview
Because there are various levels of high availability that can be deployed in enterprise branch offices, you need to identify the level you want to achieve. You can then deploy the appropriate level of device and link redundancy that supports your high availability requirements.
Link-level redundancy essentially requires two links to operate in an active/active or active/backup setting so that if one link fails, the other can take over and restore traffic forwarding that had been previously sent over the failed link. Failure on any given access link should not result in a loss of connectivity. This applies only to branch offices with at least two upstream links that are connected either to a private network or to the Internet.
Another level of high availability is device-level redundancy, effectively doubling up on devices to ensure that a backup device can take over in the event of a failed device. Typically, link redundancy and device redundancy are coupled, which means that if one fails, the other fails. With this strategy, no single device failure should result in a loss of connectivity from the branch office to the data centers.
A true high availability design that provides assured connectivity to business-critical enterprises and branch offices employs a combination of link and device redundancy that connects the branch office to dual data centers. Traffic from the branch office is dual-homed to each data center so that in the event of a complete failure in one of the data centers, traffic can be rerouted to a backup data center. Whenever failures occur (link, device, or data center), traffic should be rerouted in less than 30 seconds. Within this period of time, packet loss might occur. However, sessions are maintained if the user applications can withstand these failover times. Branch offices with redundant devices should provide session persistence so that in the event of a failure, established sessions are not dropped, even if the failed device was forwarding traffic.
Distributed Enterprise Connectivity Architecture Design Considerations provides information about link-level as well as device-level high availability deployment for each of the branch office network profiles in a distributed enterprise environment.