Rerouting Of Inter-Area Lsps - Nokia 7705 SAR-W Series Manual

MPLS Guide
3.5.5.1
In prior implementations, an inter-area LSP path would have been rerouted if a failure
or a topology change occurred in the local area or in a remote area while the ABR
loose hop in the path definition was still up. If the transit/inter-area (exit) ABR failed
or was put into node TE graceful shutdown, or if IS-IS went into overload mode, the
LSP path would remain down at the ingress LER.
With automatic ABR selection, the ingress LER can reroute an inter-area LSP
primary path via a different ABR in the following situations:
3HE 18686 AAAB TQZZA
6. Note the following special case of a multi-segment, inter-area LSP. If CSPF hits
a hop that can be reached via an intra-area path but that resides on an ABR,
CSPF only calculates a path up to that ABR. This is because there is a better
chance to reach the destination of the LSP by first signaling the LSP up to that
ABR and continuing the path calculation from there on by having the ABR
expand the remaining hops in the ERO.
7. If there is more than one ABR that advertised a prefix, CSPF calculates a path
for all ABRs. Only the shortest path is withheld. If more than one path is the
shortest path, CSPF picks a path randomly or based on the least-fill criterion if
least-fill is enabled. If more than one ABR satisfies the least-fill criterion, CSPF
also picks one path randomly.
8. The path for an intra-area LSP cannot exit and re-enter the local area of the
ingress LER. This behavior was possible in prior implementations when the user
specified a loose hop outside the local area or when the only available path was
via TE links outside the local area.

Rerouting of Inter-area LSPs

• When the local exit ABR fails, there are two cases to consider:
- If the primary path is not protected at the ABR, and is therefore torn down
by the previous hop in the path, then the ingress LER retries the LSP
primary path via the ABR that currently has the best path for the destination
prefix of the LSP.
- If the primary path is protected at the ABR with a manual or dynamic bypass
LSP, the ingress LER will receive a PathErr message with a notification of
protection becoming active downstream and a RESV message with a Local-
Protection-In-Use flag set. At the receipt of the first of these two messages,
the ingress LER performs a Global Revertive MBB procedure to optimize
the LSP primary path via the ABR that currently has the best path for the
destination prefix of the LSP.
Use subject to Terms available at: www.nokia.com
© 2022 Nokia.
MPLS and RSVP-TE
63