On an M20 router with two Routing Engines installed, you should determine whether the Routing Engine you are removing is the master. If it is, switch mastership to the other Routing Engine before removing the current master. To check and switch mastership, follow this procedure:
Packet forwarding halts while the standby Routing Engine becomes the master and resets the Packet Forwarding Engine.
If the Routing Engines are running JUNOS Release 6.0 or later and are configured for graceful switchover, the standby Routing Engine immediately assumes Routing Engine functions and there is no interruption to packet forwarding. Otherwise, packet forwarding halts while the standby Routing Engine becomes the master and the Packet Forwarding Engine components reset and connect to the new master Routing Engine. For information about configuring graceful switchover, see the section about Routing Engine redundancy in the JUNOS System Basics Configuration Guide.
Router performance might change if the standby 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 a Routing Engine, such as the hostname defined at the [edit system] hierarchy level and the management interface (fxp0 or equivalent) defined at the [edit interfaces] hierarchy level.
To configure Routing Engine-specific parameters, and still use the same configuration on both Routing Engines, 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 System Basics Configuration Guide.