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Configuring Nonstop Bridging on Switches (CLI Procedure)


This task uses switches with support for the Enhanced Layer 2 Software (ELS) configuration style. If your switch runs software that does not support ELS, see Configuring Nonstop Bridging on EX Series Switches (CLI Procedure). For ELS details, see Using the Enhanced Layer 2 Software CLI.

You can configure nonstop bridging (NSB) to provide resilience for Layer 2 protocol sessions on a Juniper Networks EX Series switch with multiple Routing Engines or an EX Series or QFX Series switch in a Virtual Chassis or Virtual Chassis Fabric configuration. Limited support for NSB is also provided on QFX5100 and EX4600 standalone switches, but NSB is enabled only during an ISSU.

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 they are already synchronized on the backup Routing Engine. The Routing Engine switchover is transparent to neighbor devices, which do not detect any changes related to the Layer 2 protocol sessions. The neighboring devices and other devices on the network do not, therefore, have to resynchronize their Layer 2 protocol states to respond to the downtime on the switch—a process that adds network overhead and risks disrupting network performance—when a Routing Engine switchover occurs when NSB is enabled.


If you are using a QFX5100 or EX4600 standalone switch and you want to use ISSU, configure Graceful Routing Engine switchover (GRES), NSB and nonstop active routing (NSR). You must configure NSB, GRES, and NSR in order to run ISSU. However, GRES, NSB and NSR are enabled only during the upgrade. During an ISSU, the Junos OS runs in two separate virtual machines (VMs)—one VM is in the primary role acting as the primary Routing Engine, and the other VM is in the backup role acting as the backup Routing Engine. The Junos OS is upgraded on the backup VM. After a successful software upgrade, the backup VM then becomes the primary VM, and the original primary VM is no longer needed and is shut down.

To configure NSB:

  1. Enable graceful Routing Engine switchover (GRES):
  2. Enable NSB:
  3. Synchronize configuration changes between the Routing Engines:

    If you try to commit a configuration that includes NSB without including the commit synchronize statement, the commit fails.


There is no requirement to start the two Routing Engines simultaneously. If the backup Routing Engine is not up when you use the commit synchronize statement, the candidate configuration is committed in the primary Routing Engine. When the backup Routing Engine comes online, its configuration is automatically synchronized with that of the primary.

Best Practice:

After a graceful Routing Engine switchover, we recommend that you issue the clear interface statistics (interface-name | all) command to reset the cumulative values for local statistics on the new primary Routing Engine.