Behavior Of Mpls Options In Inter-Area Lsps - Nokia 7705 SAR-W Series Manual

MPLS and RSVP-TE
3.5.5.2
The automatic ABR selection for an inter-area LSP does not change the prior
implementation of inter-area LSP behavior for many of the LSP-level and path-level
options. However, there are a number of enhancements introduced by the automatic
ABR selection feature.
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• When the local exit ABR node goes into IS-IS overload or is put into node TE
graceful shutdown, the ingress LER performs an MBB procedure to optimize the
LSP primary path via the ABR that currently has the best path for the destination
prefix of the LSP. The MBB is performed at the receipt of the PathErr message
for the node TE shutdown, or at the next timer or manual optimization of the LSP
path if the IS-IS overload bit is received.

Behavior of MPLS Options in Inter-area LSPs

• Features such as path bandwidth reservation and admin-groups continue to
operate within the scope of all areas since they rely on propagating the
parameter information in the PATH message across the area boundary.
• The TE graceful shutdown feature continues to support MBB of the LSP path to
avoid the link or node that originated the PathErr message as long as the link or
node is in the local area of the ingress LER. If the PathErr originated in a remote
area, the ingress LER is not able to avoid the link or node when it performs the
MBB since it computes the path to the local exit ABR only. However, there is an
exception to this. An enhancement has been added to cause the upstream
ABRs in the current path of the LSP to record the link or node to avoid and use
the record in subsequent ERO expansions. This means that if the ingress LER
computes a new MBB path that goes through the same exit ABR as the current
path, and all ABRs upstream of the node or link that originated the PathErr
message are also selected in the new MBB path when the ERO is expanded,
then the new path will also avoid this link or node.
• MBB support has been expanded to avoid the ABR when the node is put into TE
graceful shutdown.
• The use-te-metric option in CSPF cannot be propagated across the area
boundary and therefore operates within the scope of the local area of the ingress
LER. This is a new behavior.
• The srlg option on the bypass LSP continues to operate locally at each PLR
within each area. The PLR protecting the ABR checks the SRLG constraint for
the path of the bypass within the local area.
• The srlg option on the secondary path is allowed to operate within the scope of
the local area of the ingress LER with the automatic ABR selection feature.
• 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 in the local area.
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MPLS Guide
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