Alcatel-Lucent 7450 - Figure 32: Automatic ABR Node Selection for Inter-Area LSP

MPLS and RSVP
7450 ESS MPLS Guide Page 143
Figure 32: Automatic ABR Node Selection for Inter-Area LSP
CSPF for an inter-area LSP operates as follows:
1. 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 (i.e., 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.
2. When the path of the LSP is empty, CPSF will compute a single-segment intra-area path
to an ABR node that advertised a prefix matching with the destination address of the LSP.
3. When the path of the LSP contains one or more hops, CSPF will compute a multi-segment
intra-area path including the hops that are in the area of the Ingress LER node.
4. 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.
5. 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.
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.
This behavior can be illustrated in the following example. 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
ABR1_2
P2–Merge Point
P1–PLR
[S] Strict Hop
[L] Loose Hop
ABR2_1
PE1 PE2
al_0241
P3
P2
P1
ABR1_1
ERO: P2 [S], ABR2_1 [S], PE2 [L]
LSP Path Empty
Computed ERO: P1[S], ABR1_1 [S], PE2 [L]
ERO: P3 [S], PE2 [S]