M40e Host Module Description
The host module constructs routing tables, performs system management functions, and generates the SONET/SDH clock signal for SONET/SDH interfaces. It consists of a paired Routing Engine and Miscellaneous Control Subsystem (MCS).
For a host module to function, both of its components—Routing Engine and MCS—must be installed and operational. One or two host modules can be installed into the midplane from the rear of the chassis, as shown in M40e Chassis Description. The Routing Engine slot labeled RE 0 is below the MCS slot labeled MCS 0 and the RE 1 slot is above the MCS 1 slot.
If two host modules are installed, both are powered on, but only one is active (the master); the second host module is in standby mode and performs no functions. By default, the master host module is the one with components installed in the RE 0 and MCS 0 slots. To change the default master Routing Engine, include the appropriate [edit chassis redundancy routing-engine] statement in the configuration, as described in the section about Routing Engine redundancy in the Junos System Basics Configuration Guide.
The host module components are hot-pluggable, as described in M40e Field-Replaceable Units (FRUs). Removal or failure of one or both components in the standby host module does not affect router function. Removal or failure of one or both components in the master host module 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.
If you do not configure graceful Routing Engine switchover, graceful restart, or nonstop active routing, you can configure automatic Routing Engine mastership failover. For information about configuring automatic mastership failover, see the Junos System Basics Configuration Guide.
 | Note: 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. |
| Note: For information about configuring graceful Routing Engine switchover, graceful restart, and nonstop active routing, see the Junos High Availability Configuration Guide. |
| Note: The first supported release for graceful Routing Engine switchover and nonstop active routing on the M40e router is Junos OS Release 5.7 and Junos OS Release 8.4, respectively. However, for graceful Routing Engine switchover we recommend Junos OS Release 7.0 or later. Graceful restart software requirements are dependent on the routing protocols configured on the router. For the minimum software requirements for graceful restart, see the Junos High Availability Configuration Guide. |
For host module replacement instructions, see Replacing an MCS in an M40e Router and Replacing a Routing Engine in an M40e Router.
Figure 1: M40e and M160 Router Host Module Location
On M40e and M160 routers, the host module consists of a paired Routing Engine and MCS. One pair functions as master, while the other stands by as a backup should the master Routing Engine fail. (See Replacing an MCS in an M40e Router and Replacing a Routing Engine in an M40e Router.)
Figure 2: M40e and M160 Router Redundant Host Modules
