This article explains why the former primary Routing Engine (RE) restarts after an RE switchover on an MX router with Next Generation Subscriber Management configuration, and clarifies that this is expected behavior.
When there is an RE switchover on a system running with redundancy and graceful Routing Engine switchover (GRES) enabled, if the system has Next-Generation Broadband-Edge configured, the former primary RE will restart. The following log messages are seen on the former primary RE:
Oct 5 18:50:56 device eventd: sendto: No route to host Oct 5 18:50:56 device eventd[4671]: SYSTEM_ABNORMAL_SHUTDOWN: System abnormally shut down Oct 5 18:50:56 device eventd[4671]: SYSTEM_OPERATIONAL: System is operational
Also, the output of show chassis routing-engine reflects the fact that the former primary RE has restarted.
show chassis routing-engine
This is expected behavior due to changes in how state is mirrored between redundant REs in the new Subscriber Management architecture.
The Next Generation Broadband Edge architecture leverages its own mechanisms for mirroring state to the standby control planes, operating in a hot standby mechanism. So, all subscriber-related state is mirrored from the primary RE to the backup RE in real time when GRES is enabled.
In the event of an RE switch, the standby control plane, which is the old primary RE, will perform a cold restart after the switch to enable it to re-synchronize its state from the new primary control plane. The cold restart ensures that there is no stuck state or any discrepancies between the subscriber databases that could have taken place during the switchover window.
2021-06-18: "Next Generation Broadband-Edge releases" actually refer to Junos OS Release 15.1R4 and all newer Junos OS releases. The former RE will restart after GRES only if Junos Subscriber Management (JSM) is enabled. Article modified accordingly to reflect this information; also all references of primary RE changed to primary RE in compliance with Inclusion and Diversity standards implemented by Juniper