Inter-area RSVP point-to-point LSPs support automatic area border router (ABR) selection at the ingress LER. The ABR does not need to be included as a loose hop in the LSP path definition.
CSPF can now compute all segments of a multi-segment, inter-area LSP path in one operation. Previously, MPLS made separate requests to CSPF for each segment.
For LSP path establishment, the explicit route object (ERO) in the PATH message is expanded on ABRs where the next hop is a loose hop in the LSP path definition. For ERO expansion to operate, the cspf-on-loose-hop command must be enabled under the mpls context on the ABR to allow the ABR to perform a CSPF calculation. If CSPF calculations are not performed, CSPF for the LSP path fails at the head-end node as TE information for links in another area are not available.
Figure: Automatic ABR Selection for Inter-Area LSP illustrates the role of each node in the signaling of an inter-area LSP with automatic ABR selection.
CSPF for an inter-area LSP operates as follows:
CSPF in the ingress LER node determines that an LSP is inter-area by performing a route lookup with the destination address of a point-to-point LSP, such as the address in the ‟to” field of the LSP configuration. If there is no intra-area route to the destination address, the LSP is considered to be inter-area.
When the path of the LSP is empty, CPSF computes a single-segment, intra-area path to an ABR that advertised a prefix matching the destination address of the LSP.
If the path of the LSP contains one or more hops, CSPF computes a multi-segment, intra-area path including the hops that are in the area of the ingress LER node.
If all hops are in the area of the ingress LER, the calculated path ends on an ABR that advertised a prefix matching the destination address of the LSP.
When there are one or more hops that are not in the area of the ingress LER, the calculated path ends on an ABR that advertised a prefix matching the first-hop address that is not in the area of the ingress LER.
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.
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.
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.