Specifications

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connection is broken, the user can switchover mastership by typing the
“switchover” command. (More information on switching over below.) If the
master RE is hot-swapped out of the system, the backup RE will take over
the control of the system as the new master RE. Again, once an RE
becomes master, it resets the switch plane and downloads its own version of
the microkernel to the PFE components.
§ Disabled—An RE that is disabled has progressed through its memory check
and boot sequence to the point of displaying a login prompt (similar to
Backup state) but does not respond to a switchover command. An RE in
Disabled state supports full management access through the Ethernet,
console, and auxiliary ports and can communicate with the master RE. An
disabled RE does not participate in a mastership election. In order to move
from Disabled state to Backup state, the RE must be reconfigured to be
Backup.
Mastership Election
For the case of a single RE system, the RE will become the master immediately
unless it is configured as disabled. For a dual RE system, the RE’s will establish
connection between each other, through an internal Ethernet link, to resolve
mastership. To avoid duplicated masters as a result of mis-configuration (ie both
RE’s are configured as master) each RE, when configured to be master, will
monitor the latest mastership status for a short period of time before becoming
the master. If there is a master RE in the system already, it will alert the network
administrator of the mis-configuration and will automatically be placed in Backup
State. When both RE’s have the same configuration and come up at the same
time, the slot 0 RE will take precedence over the slot 1 RE. Additionally, the RE
will become master if it is configured as master and if the connection with the
other RE cannot be established.
Mastership Switchover Process
When a master RE fails and the user determines that a switchover is needed, a
switchover can be initiated using a reset command with the following parameters
:
§ acquire Attempt to become the master routing engine
§ release Request the other routing engine become master
§ switch Toggle mastership between routing engines
The acquire command is entered on the backup RE and is used specifically to
request that the backup RE attempt to become the master RE. The release
command is entered on the master and is used specifically for requesting that the
backup RE become the master. The switch command toggles the mastership
between RE and can be entered on either RE.
When the command is entered, the system will gracefully pass over mastership to
the backup RE. Specifically, the master RE will give up control of the system bus
and put itself into the backup state. The backup RE will become master and will
restart the switch plane. The original master can then be diagnosed for
problems, or prepared for upgrade/reconfiguration. When switchover occurs, the
backup RE does not have to run through its full boot cycle (see table 1). After the
switchover, the user can switch back to the original mastership immediately if
necessary. It should be noted that customers are responsible for maintaining the
configuration and software image on the backup RE. There is no support for a
synchronize configuration command in the current release of Junos; however
configurations can be copied from one RE to another.
Once the switchover has occurred, the new master must re-establish routing
adjacencies, build a routing table, and transfer forwarding table information to the
SSB. A change in RE mastership requires a rebooting of the SSB to re-establish
communication links and to download the microkernel. When this occurs,
forwarding will be interrupted and packet buffers will be flushed.
If the master RE hangs when the switchover command is being executed on the
backup RE, the RE connection will be lost. The master RE then would require a
reset to release the bus mastership to the new master. The master RE can be
reset from the backup RE, and mastership switches over.
Using RE Redundancy to Manage Software Upgrades
The RE redundancy model can be used to facilitate smooth introduction of new
software loads. With the current software image running on the Master RE, a
new software image can be loaded onto the Backup RE. The user can then issue
the switchover command to switch mastership to the backup RE. If the new