MPLS Guide MPLS and RSVP-TE
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All other feature behavior and limitations are the same as for an auto-created LSP
mesh.
3.5.5 Automatic ABR Selection for Inter-area LSPs
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 6 illustrates the role of each node in the signaling of an inter-area LSP with
automatic ABR selection.
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