Understanding Nonstop Bridging

Nonstop bridging uses the same infrastructure as graceful Routing Engine switchover (GRES) to preserve interface and kernel information. However, nonstop bridging also saves Layer 2 Control Protocol (L2CP) information by running the Layer 2 Control Protocol process (l2cpd) on the backup Routing Engine.

Figure 1 shows the system architecture of nonstop bridging and the process a routing (or switching) platform follows to prepare for a switchover.

Figure 1: Nonstop Bridging Switchover Preparation Process

The switchover preparation process for nonstop bridging follows these steps:

1. The primary Routing Engine starts.

2. The routing platform processes on the primary Routing Engine (such as the chassis process [chassisd] and the Layer 2 Control Protocol process [l2cpd]) 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 chassis process (chassisd) and the Layer 2 Control Protocol process (l2cpd).

6. The system determines whether graceful Routing Engine switchover and nonstop bridging have been enabled.

7. The kernel synchronization process (ksyncd) synchronizes the backup Routing Engine with the primary Routing Engine.

8. For supported protocols, state information is updated directly between the l2cpds on the primary and backup Routing Engines.

Figure 2 shows the effects of a switchover on the routing platform.

Figure 2: Nonstop Bridging During a Switchover

The switchover process follows these 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 Layer 2 Control Protocol process (l2cpd) 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 bridging are continued during the switchover, resulting in minimal packet loss.

You can configure nonstop bridging (NSB) to provide resilience for Layer 2 protocol sessions on a Juniper Networks EX Series Ethernet Switch or on an EX Series Virtual Chassis with redundant Routing Engines.

NSB operates by synchronizing all protocol information for NSB-supported Layer 2 protocols between the primary and backup Routing Engines. If the switch has a Routing Engine switchover, the NSB-supported Layer 2 protocol sessions remain active because all session information is already synchronized to the backup Routing Engine. Traffic disruption for the NSB-supported Layer 2 protocol is minimal or nonexistent as a result of the switchover. The Routing Engine switchover is transparent to neighbor devices, which do not detect any changes related to the NSB-supported Layer 2 protocol sessions on the switch.

For a list of the EX Series switches and Layer 2 protocols that support NSB, see EX Series Switch Software Features Overview and EX Series Virtual Chassis Software Features Overview.

This topic contains the following sections:

• Platform Support
• Protocol Support