Default High Availability Feature

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The high availability feature that is enabled by default on Junos devices is nonstop forwarding (NSF).

Nonstop forwarding (NSF) is a Junos feature that ensures continuous packet forwarding during a Routing Engine switchover. When a Routing Engine switchover occurs due to a failure or a planned maintenance activity, the backup Routing Engine takes over control and begins forwarding packets without interruption.

During an NSF switchover, the new Routing Engine takes over and continues to forward packets using the same forwarding table and the same routing protocol adjacencies as the failed Routing Engine. This ensures that there is no disruption to network traffic during the switchover process.

NSF is enabled by default on Junos devices, meaning that it is always active and ready to ensure continuous packet forwarding in the event of a Routing Engine switchover. Other high availability features, such as graceful Routing Engine switchover (GRES) and nonstop active routing (NSR), need to be explicitly configured to be enabled.

Grateful Routing Engine Switchover (GRES) is a feature that enables fast failover from one Routing Engine to another. When GRES is enabled, the backup Routing Engine keeps a copy of the forwarding table and routing protocol adjacencies in memory, so it can take over quickly if the primary Routing Engine fails.

Root protection is a feature that helps prevent loops in Spanning Tree Protocol (STP) networks by blocking ports that could cause a loop. Root protection is not a high availability feature, but it is important for network stability.

In summary, nonstop forwarding (NSF) is the high availability feature that is enabled by default on Junos devices. It ensures continuous packet forwarding during a Routing Engine switchover, and it helps to minimize network downtime.