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Replacing T4000 Host Subsystem Components

  1. Taking the T4000 Host Subsystem Offline

  2. Replacing a Control Board or Routing Engine Component

Taking the T4000 Host Subsystem Offline

Before you replace a control board or Routing Engine,, you must take the host subsystem offline. The host subsystem is taken offline and brought online as a unit. Normally, if two host subsystems are installed in the router, RE0 functions as the master and RE1 functions as the backup.


Router performance might change if the backup Routing Engine's configuration differs from the former master's configuration. For the most predictable performance, configure the two Routing Engines identically, except for parameters unique to each Routing Engine.


For information about high availability features, configuring graceful switchover and nonstop active routing, see the Junos OS High Availability Library for Routing Devices.

Table 1 explains the effect of taking the host subsystem offline.

Table 1: Effect of Taking the Host Subsystem Offline

Type of Host Subsystem

Effect of taking the Host Subsystem Offline

Nonredundant host subsystem

The router shuts down.

Backup host subsystem

The functioning of the router is not interrupted. The backup host subsystem is hot-removable and hot-insertable.

Master host subsystem

The backup host subsystem becomes the master. The backup Routing Engine assumes Routing Engine functions. The master host subsystem is hot-pluggable. Removal or failure of the master Routing Engine affects forwarding and routing based on the high availability configuration:

  • Dual Routing Engines without any high availability features enabled—Traffic is interrupted while the Packet Forwarding Engine is reinitialized. All kernel and forwarding processes are restarted. When the switchover to the new master Routing Engine is complete, routing convergence takes place and traffic is resumed.

  • Graceful Routing Engine switchover (GRES) is enabled—Graceful Routing Engine switchover preserves interface and kernel information. Traffic is not interrupted. However, graceful Routing Engine switchover does not preserve the control plane. Neighboring routers detect that the router has restarted and react to the event in a manner prescribed by individual routing protocol specifications. To preserve routing without interruption during a switchover, graceful Routing Engine switchover must be combined with nonstop active routing.

    Note: Graceful Routing Engine switchover (GRES) is supported on Junos OS Release 12.1R2 and later for T4000 routers.

  • Nonstop active routing is enabled (graceful Routing Engine switchover must be configured for nonstop active routing to be enabled)—Nonstop active routing supports Routing Engine switchover without alerting peer nodes that a change has occurred. Nonstop active routing uses the same infrastructure as graceful Routing Engine switchover to preserve interface and kernel information. However, nonstop active routing also preserves routing information and protocol sessions by running the routing protocol process (rpd) on both Routing Engines. In addition, nonstop active routing preserves TCP connections maintained in the kernel.

    Nonstop active routing is supported on Junos OS Release 12.1R2 and later for T4000 routers,

  • Graceful restart is configured—Graceful restart provides extensions to routing protocols so that neighboring helper routers restore routing information to a restarting router. These extensions signal neighboring routers about the graceful restart and prevent the neighbors from reacting to the router restart and from propagating the change in state to the network during the graceful restart period. Neighbors provide the routing information that enables the restarting router to stop and restart routing protocols without causing network reconvergence. Neighbors are required to support graceful restart. The routing protocol process (rpd) restarts. A graceful restart interval is required. For certain protocols, a significant change in the network can cause graceful restart to stop.

Replacing a Control Board or Routing Engine Component


To replace a solid-state disk or CompactFlash card, you must power off the Routing Engine.

After you take the host subsystem offline, you can replace the following host subsystem components:

See also