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MPLS Guide
3.5.5.3
The OSPF virtual link extends area 0 for a router that is not connected to area 0
(OSPF backbone area). All prefixes in area 0 appear to be reachable via an intra-
area path. However, the prefixes are not reachable since the path crosses the transit
area through which the virtual link is set up to reach the area 0 remote nodes.
The TE database in a router learns all of the remote TE links in area 0 from the ABR
connected to the transit area, but an intra-area LSP path using these TE links cannot
be signaled within area 0 since none of these links are directly connected to this
node.
The inter-area LSP feature can identify when the destination of an LSP is reachable
via a virtual link. In that case, CSPF automatically computes and signals an inter-
area LSP via the ABRs that are connected to the transit area.
However, when the ingress LER for the LSP is the ABR connected to the transit area,
and the destination of the LSP is the address corresponding to another ABR router-
id in that same transit area, CSPF computes and signals an intra-area LSP using the
transit area TE links, even when the destination router-id is only part of area 0.
3.5.6 ABR FRR Protection for Inter-area LSP
For protection of the ABR, the upstream node of the ABR acts as a PLR, and the
next-hop node to the protected domain border router is the merge point (MP). Both
manual and dynamic bypass are available to protect the ABR.
Manual bypass protection only works when a proper completely strict path is
provisioned that avoids the ABR.
Dynamic bypass protection provides for the automatic computation, signaling, and
association with the primary path of an inter-area point-to-point LSP to provide ABR
protection.
dynamic bypass LSP.
3HE 18686 AAAB TQZZA
• The PLR must indicate to CSPF that a request to a one-to-one detour LSP path
must remain within the local area. If the destination for the detour, which is the
same as that of the LSP, is outside of the area, CSPF must return no path.
• With the automatic ABR selection feature, timer-based resignaling of the inter-
area LSP path is supported and the path is resignaled if the cost of the path
segment to the local exit ABR changes. The cost shown for the inter-area LSP
at the ingress LER is the cost of the path segments to the ABR.
Inter-area LSP Support of OSPF Virtual Links
Figure 7
illustrates the role of each node in ABR protection using a
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