Inter-Area Lsp Support Of Ospf Virtual Links; Area Border Node Frr Protection For Inter Area Lsp - Alcatel-Lucent 7450 Manual

Inter Area TE LSP (ERO Expansion Method)
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Inter-Area LSP support of OSPF Virtual Links

The OSPF virtual link extends area 0 for a router that is not connected to area 0. As a result, it
makes all prefixes in area 0 reachable via an intra-area path but in reality, they are not 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 is directly connected to this node.
This inter-area LSP feature can identify when the destination of an LSP is reachable via a virtual
link. In that case, CSPF will automatically compute and signal an inter-area LSP via the ABR
nodes that is 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 will compute and signal an intra-area LSP using the transit area TE links, even when
the destination router-id is only part of area 0.

Area Border Node FRR Protection for Inter Area LSP

For protection of the area border router, the upstream node of the area border router acts as a point-
of-local-repair (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 area border node.
Manual bypass protection works only when a proper completely strict path is provisioned that
avoids the area border node.
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The least-fill option support with an inter-area LSP is introduced with the automatic ABR
selection feature. When this option is enabled, CSPF applies the least-fill criterion to
select the path segment to the exit ABR node in the local area.
1The PLR node must indicate to CSPF that a request to 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.
The propagate-admin-group option under the LSP will still need to be enabled on the
inter-area LSP if the user wants to have admin-groups propagated across the areas.
With the automatic ABR selection feature, timer based re-signal of the inter-area LSP path
will be supported and will re-signal the path if the cost of the path segment to the local exit
ABR changed. The cost shown for the inter-area LSP at ingress LER will be the cost of the
path segments to the ABR node.
7450 ESS MPLS Guide