Nonstop Active Routing Concepts

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

Note: To use nonstop active routing, you must first enable graceful Routing Engine switchover on your routing platform. For more information about graceful Routing Engine switchover, see Graceful Routing Engine Switchover Overview.

Figure 5 shows the system architecture of nonstop active routing and the process a routing platform follows to prepare for a switchover.

Figure 5: Nonstop Active Routing Switchover Preparation Process

The switchover preparation process for nonstop active routing follows these steps:

1. The master Routing Engine starts.
2. The routing platform processes on the master Routing Engine (such as the chassis process [chassisd] and the routing protocol process [rpd]) start.
3. The Packet Forwarding Engine starts and connects to the master Routing Engine.
4. All state information is updated in the system.
5. The backup Routing Engine starts, including the chassis process (chassisd) and the routing protocol process (rpd).
6. The system determines whether graceful Routing Engine switchover and nonstop active routing have been enabled.
7. The kernel synchronization process (ksyncd) synchronizes the backup Routing Engine with the master Routing Engine.
8. For supported protocols, state information is updated directly between the routing protocol processes on the master and backup Routing Engines.

Figure 6 shows the effects of a switchover on the routing platform.

Figure 6: Nonstop Active Routing During a Switchover

The switchover process follows these steps:

1. When keepalives from the master 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 master. Because the routing protocol process (rpd) and chassis process (chassisd) are already running, these 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 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.