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    Effect of Taking the MX2020 Host Subsystem Offline

    The host subsystem is taken offline and brought online as a unit. Before you replace a Control Board and Routing Engine (CB-RE) , you must take the host subsystem offline. The host subsystem is hot-pluggable.

    Normally, if two host subsystems are installed in the router, one CB-RE functions as the master and the other as back-up. You can remove the backup host subsystem (or either of its components) without interrupting the functioning of the router. If you take the master host subsystem offline, the backup host subsystem becomes the master. Depending on the configuration, the individual FRUs may reboot upon switching mastership. If the router has only one host subsystem, taking the host subsystem offline causes the router to shut down. The effect of taking the master host subsystem offline varies depending on your configuration of high availability features.

    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 CB-RE assumes routing engine functions. The master host subsystem is hot-pluggable. Removal or failure of the master CB-RE affects forwarding and routing based on the high availability configuration:

    • Dual CB-RE 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 CB-RE is complete, routing convergence takes place and traffic is resumed.
    • Graceful CB-RE switchover is enabled—Graceful CB-RE switchover preserves interface and kernel information. Traffic is not interrupted. However, graceful CB-RE 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 CB-RE switchover must be combined with nonstop active routing.
    • Nonstop active routing is enabled (graceful CB-RE switchover must be configured for nonstop active routing to be enabled)—Nonstop active routing supports CB-RE switchover without alerting peer nodes that a change has occurred. Nonstop active routing uses the same infrastructure as graceful CB-RE 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 CB-REs. In addition, nonstop active routing preserves TCP connections maintained in the kernel.
    • 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.

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

    To configure the Routing Engine on the Control Board (CB-RE)—specific parameters and still use the same configuration on both Routing Engines on the Control Board, include the appropriate configuration statements under the re0 and re1 statements at the [edit groups] hierarchy level, and use the apply-groups statement. For instructions, see the Junos OS Administration Library.

    Note: For information about configuring graceful Routing Engine on the CB-RE switchover, graceful restart, and nonstop active routing, see the Junos OS High Availability Library for Routing Devices.

    Note: The first supported release for both graceful Routing Engine on the CB-RE switchover and nonstop active routing on the router is Junos OS Release 12.3R2. Graceful restart software requirements depend on the routing protocols configured on the router. For the minimum software requirements for graceful restart, see the Junos OS High Availability Library for Routing Devices.

    Modified: 2016-03-29