Host Module

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 Figure 3: 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 Internet Software Configuration Guide: Getting Started.

The host module components are hot-pluggable, as described in Field-Replaceable Units (FRUs). Removal or failure of one or both components in the standby host module does not affect router function. If one or both components in the master host module is removed from the chassis, the effect depends on whether two host modules are installed:

• If there is only one host module, packet forwarding halts until both the Routing Engine and MCS are reinstalled and functioning normally.
• If there are two host modules, the effect depends on the software configuration:
  • If the Routing Engines are running JUNOS Release 6.0 or later and are configured for graceful switchover, the standby Routing Engine automatically assumes mastership without interruption of forwarding performance. For information about configuring graceful switchover, see the section about Routing Engine redundancy in JUNOS Internet Software Configuration Guide: Getting Started.
  • Otherwise, forwarding halts while standby host module becomes the master and the new master Routing Engine resets the Packet Forwarding Engine.

For host module replacement instructions, see Replacing an MCS and Replacing a Routing Engine.

Note that the effect of a hardware or software failure on one or both components in the master host module differs from the effect of removing a component that belongs to the master host module:

• With the default router configuration, in case of failure you must correct the problem manually. You can issue the appropriate request chassis routing-engine master command to switch mastership to the other Routing Engine, for example. For information about the command, see the JUNOS Internet Software Operational Mode Command Reference: Protocols, Class of Service, Chassis, and Management.
• On routers with two installed Routing Engines running JUNOS Release 6.0 or later, you can configure graceful switchover of Routing Engines, as previously described for the case of Routing Engine removal. When the standby Routing Engine stops receiving keepalive signals from the master Routing Engine, it automatically assumes mastership without interruption of forwarding performance.
• On routers with two installed Routing Engines running any JUNOS release, you can configure automatic Routing Engine mastership failover. When the standby Routing Engine stops receiving keepalive signals from the master Routing Engine, it automatically assumes mastership. Packet forwarding halts while the Packet Forwarding Engine components reset and connect to the new master Routing Engine.

For information about configuring graceful switchover or automatic mastership failover, see the section about Routing Engine redundancy in the JUNOS Internet Software Configuration Guide: Getting Started.

For more information about host module components, see the following sections:

• Routing Engine
• Miscellaneous Control Subsystem (MCS)