MPLS Guide MPLS and RSVP-TE
3HE 18686 AAAB TQZZA © 2022 Nokia.
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When SIDs are explicitly configured for a path, the user must provide all of the
necessary SIDs to reach the destination. The router does not validate whether the
whole label stack provided is correct other than checking that the top SID is
programmed locally. A path can come up even if it contains SIDs that are invalid. The
user or controller programming the path should ensure that the SIDs are correct.
A path must consist of either all SIDs or all IP address hops.
Paths containing explicit SID values can only be used by SR-TE LSPs.
3.12.5 SR-TE LSP Protection
The router supports local protection of a particular segment of an SR-TE LSP and
end-to-end protection of the complete SR-TE LSP.
Each path is locally protected along the network using LFA or remote-LFA next hop
whenever possible. The protection of a node SID reuses the LFA and remote LFA
features introduced with segment routing shortest path tunnels; the protection of an
adjacency SID has been added to the 7705 SAR in the specific context of an SR-TE
LSP to augment the protection level. The user must enable the loopfree-alternates>
remote-lfa command in IS-IS or OSPF.
An SR-TE LSP has state at the ingress LER only. The LSR has state for the node
SID and adjacency SID, whose labels are programmed in the label stack of the
received packet and which represent the part of the ERO of the SR-TE LSP on this
router and downstream of this router. In order to provide protection for an SR-TE
LSP, each LSR node must attempt to program a link-protect or node-protect LFA
next hop in the ILM record of a node SID or an adjacency SID, and the LER node
must do the same in the LTN record of the SR-TE LSP. The following are details of
the behavior.
• If the ILM record is for a node SID of a downstream router that is not directly
connected, the ILM of this node SID points to the backup NHLFE computed by
the LFA SPF and programmed by the SR module for this node SID. Depending
on the topology and LFA policy used, this can be a link-protect or node-protect
LFA next hop.
This behavior is already supported in the SR shortest path tunnel feature at both
the LER and LSR. Therefore, an SR-TE LSP that transits at an LSR and that
matches the ILM of a downstream node SID automatically takes advantage of
this protection when enabled. If required, node SID protection can be disabled
under the IGP instance by excluding the prefix of the node SID from the LFA.
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