Normally, if two Routing Engines are installed in the router, RE 0 functions as the master
and RE 1 functions as the backup. You can remove the backup Routing Engine (or either
of its components) without interrupting the functioning of the router. If you take the
master Routing Engine offline, the backup Routing Engine becomes the master (the
router might reboot, depending on your configuration). If the router has only one Routing
Engine, taking the Routing Engine offline causes the router to shut down. The effect of
taking the master Routing Engine offline varies depending on your configuration of high
availability features.
Table 74 on page 153 explains the effect of taking the Routing Engine offline.
Table 74: Effect of Taking the Routing Engine Offline
Effect of Taking the Routing Engine OfflineType of Routing Engine
The router shuts down.Nonredundant Routing
Engine
The functioning of the router is not interrupted. The backup Routing Engine is hot-removable and
hot-insertable.
Backup Routing Engine
The backup Routing Engine becomes the master. The backup Routing Engine assumes Routing
Engine functions. The master Routing Engine 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.
•
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.
•
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.
Master Routing Engine
153Copyright © 2017, Juniper Networks, Inc.
Chapter 21: Replacing Host Subsystem Components
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