If a BGP session protected by LLGR goes down because of a restart or failure of the peer, then the SR OS router activates GR+LLGR helper mode for all the protected AFI/SAFI. In GR+LLGR helper mode, the received routes of a particular AFI/SAFI are retained as stale routes for a maximum duration of:
restart-time + LLGR-stale-time
where:
restart-time is signaled in the GR capability of the peer (but overridden, if necessary, by the locally-configured helper-override-restart-time command).
LLGR-stale-time is signaled in the LLGR capability of the peer (but overridden, if necessary, by a locally-configured helper-override-stale-time command).
While the restart-timer is running, the SR OS router acts in the normal GR helper role. When the restart-timer elapses, the LLGR phase begins. When LLGR starts, the following tasks occur.
The LLGR-stale-time starts to count down.
Stale routes marked with the NO_LLGR community are immediately deleted.
Remaining stale routes are not preferred. The BGP best path selection algorithm is rerun with a new first step that prefers valid, non-stale LLGR routes over any stale LLGR routes.
If a de-preferenced stale route remains, the best and valid NH-reachable path for the NLRI is re-advertised, with an added LLGR_STALE community, to peers that signaled support for the LLGR capability. The route may be withdrawn or re-advertised toward peers that do not support LLGR, subject to the configuration of the advertise-stale-to-all-neighbors command and without-no-export parameter.
LLGR ends for a particular AFI/SAFI when the LLGR-stale-time reaches zero. At that time, all remaining stale routes of the AFI/SAFI are deleted. The LLGR-stale-time is not stopped by re-establishment of the session with the failed peer; it continues until the EoR marker is received for the AFI/SAFI.
Stale routes may be deleted before the expiration of the LLGR-stale-time. If the session with the failed peer comes back up and one of the following is true, then the stale routes should be deleted immediately.
The GR or LLGR capability is missing.
The AFI/SAFI is missing from the LLGR capability.
The F bit is equal to 0 for the AFI/SAFI.