This feature enhances the prior implementation of an inter-area RSVP P2P LSP by making the ABR selection automatic at the ingress LER. The user does not need to include the ABR as a loose-hop in the LSP path definition.
CSPF adds the capability to compute all segments of a multi-segment intra-area or inter-area LSP path in one operation.
Figure: Automatic ABR node selection for inter-area LSP illustrates the role of each node in the signaling of an inter-area LSP with automatic ABR node selection.
CSPF for an inter-area LSP operates as follows:
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CSPF in the Ingress LER node determines that an LSP is inter-area by doing a route lookup with the destination address of a P2P LSP (that is 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 as inter-area.
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When the path of the LSP is empty, CSPF computes a single-segment intra-area path to an ABR node that advertised a prefix matching with the destination address of the LSP.
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When 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.
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When all hops are in the area of the ingress LER node, the calculated path ends on an ABR node that advertised a prefix matching with the destination address of the LSP.
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When there are one or more hops that are not in the area of the ingress LER node, the calculated path ends on an ABR node that advertised a prefix matching with the first hop-address that is not in the area of the ingress LER node.
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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.
This behavior can be illustrated in the Figure: CSPF for an inter-area LSP. The TE link between ABR nodes D and E is in area 0. When node C computes the path for LSP from C to B which path specified nodes C and D as loose hops, it would fail the path computation if CSPF attempted a path all the way to the last hop in the local area, node E. Instead, CSPF stops the path at node A which further expands the ERO by including link D-E as part of the path in area 0.
Figure: CSPF for an inter-area LSP
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If there is more than 1 ABR that advertised a prefix, CSPF calculates a path for all ABRs. Only the shortest path is withheld. If more than one path has the shortest path, CSPF picks a path randomly or based on the least-fill criterion if enabled. If more than one ABR satisfies the least-fill criterion, CSPF also picks one path randomly.
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The path for an intra-area LSP path is not able to exit and re-enter the local area of the ingress LER. This behavior was possible in prior implementation when the user specified a loose hop outside of the local area or when the only available path was via TE links outside of the local area.