Nonstop Active Routing Concepts for Junos OS Evolved

Nonstop active routing (NSR) uses the same infrastructure as graceful Routing Engine switchover (GRES) to preserve interface and kernel information. However, NSR also synchronizes routing protocol information by running the routing protocol process (rpd) on the backup Routing Engine. By synchronizing this additional information, NSR is self-contained and does not rely on helper routers (or switches) to assist the routing platform in restoring routing protocol information. NSR is advantageous in networks in which neighbor routers (or switches) do not support graceful restart protocol extensions. As a result of this enhanced functionality, NSR is a natural replacement for graceful restart.

To activate NSR, use the set routing-options nonstop-routing configuration statement.

The switchover preparation process for NSR comprises the following steps:

1. The primary Routing Engine starts.

2. The routing platform processes on the primary Routing Engine (such as the routing protocol process [rpd]) start.

3. The Packet Forwarding Engine starts and connects to the primary Routing Engine.

4. All state information is updated in the system.

5. The backup Routing Engine starts, including the routing protocol process (rpd).

6. The system determines whether GRES and NSR have been enabled.

7. The backup Routing Engine is synchronized with the primary Routing Engine.

8. For supported protocols, state information is updated directly between the routing protocol processes on the primary and backup Routing Engines.

The switchover process comprises the following steps:

1. When keepalives from the primary Routing Engine are lost, the system switches over gracefully to the backup Routing Engine.

2. The Packet Forwarding Engine connects to the backup Routing Engine, which becomes the new primary. Because the routing protocol process (rpd) is already running, this processes do not need to restart.

3. State information learned from the point of the switchover is updated in the system. Forwarding and routing are continued during the switchover, resulting in minimal packet loss.

4. Peer routers or switches continue to interact with the routing platform as if no change had occurred. Routing adjacencies and session state relying on underlying routing information are preserved and not reset.

To verify readiness, issue the show system switchover operational mode command.