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MPLS Guide
3.12.10 Data Path Support
The support of SR-TE in the data path requires that the ingress LER push a label
stack where each label represents a hop, a TE link, or a node, in the ERO for the LSP
path computed by the router or the PCE. However, only the label and the outgoing
interface to the first strict or loose hop in the ERO factor into the forwarding decision
of the ingress LER. In other words, the SR-TE LSP only needs to track the
reachability of the first strict or loose hop.
The first strict or loose hop of the SR-TE LSP is represented as an NHFLE to the SR
shortest path tunnel. The rest of the SR-TE label stack can have a larger size and is
modeled as another NHLFE referred to as a "super NHLFE".
Therefore, an SR-TE LSP is modeled in the ingress LER data path as a hierarchical
LSP with the super NHLFE tunneled over the NHLFE of the SR shortest path tunnel
to the first strict or loose hop in the SR-TE LSP path ERO.
Some characteristics of this design are as follows.
The data path behavior at the LSR and egress LER for an SR-TE LSP is similar to
that of the shortest path tunnel because there is no tunnel state in these nodes. The
forwarding of the packet is based on processing the incoming label stack consisting
of a node SID and/or adjacency SID label. If the ILM is for a node SID and multiple
next hops exist, ECMP spraying is supported at the LSR.
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• The design saves on NHLFE usage. When many SR TE LSPs are going to the
same first hop, they will be using the same SR shortest path tunnel and will
consume one super NHLFE each, but they will be pointing to a single NHLFE,
or set of NHLFEs, when ECMP exists for the first strict or loose hop, of the
first-hop SR tunnel.
The ingress LER does not need to program a separate backup super NHLFE.
Instead, the single super NHLFE will automatically begin forwarding packets
over the LFA backup path of the SR tunnel to the first hop as soon as it is
activated.
• There is an exception to the above model in the case where the user configured
an empty path SR-TE LSP that uses the router's hop-to-label translation. In this
case, the SR-TE LSP will use the NHLFE of the node SID of the destination
router. The super NHLFE is null in this case.
• If the first segment is a node SID tunnel and multiple next hops exist, ECMP
spraying is supported at the ingress LER.
• If the first-hop SR tunnel, node SID, or adjacency SID goes down, the SR
module informs MPLS that the outer tunnel is down and MPLS brings the
SR-TE LSP down and requests the SR to delete the SR-TE LSP in the IOM.
Use subject to Terms available at: www.nokia.com
© 2022 Nokia.
MPLS and RSVP-TE
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