3.1.2. SR-MPLS

When SR is used over the MPLS data plane, SIDs are an MPLS label or an index into an MPLS label space (either SRGB or SRLB).

Where possible, it is recommended that identical SRGBs be configured on all nodes in an SR domain. This simplifies troubleshooting as the same label will be associated with the same prefix on all nodes. In addition, it simplifies support for anycast as detailed in Section 3.3.

The following behaviors are associated with SR operating over the MPLS data plane:

The IGP signaling extension for IGP-Prefix segment includes a flag to indicate whether directly connected neighbors of the node on which the prefix is attached should perform the NEXT operation or the CONTINUE operation when processing the SID. This behavior is equivalent to Penultimate Hop Popping (NEXT) or Ultimate Hop Popping (CONTINUE) in MPLS.
A Prefix-SID is allocated in the form of an MPLS label (or an index in the SRGB) according to a process similar to IP address allocation. Typically, the Prefix-SID is allocated by policy by the operator (or Network Management System (NMS)), and the SID very rarely changes.
While SR allows a local segment to be attached to an IGP prefix, where the terminology "IGP-Prefix segment" or "Prefix-SID" is used, the segment is assumed to be global (i.e., the SID is defined from the advertised SRGB). This is consistent with all the described use cases that require global segments attached to IGP prefixes.
The allocation process MUST NOT allocate the same Prefix-SID to different prefixes.
If a node learns of a Prefix-SID that has a value that falls outside the locally configured SRGB range, then the node MUST NOT use the Prefix-SID and SHOULD issue an error log reporting a misconfiguration.
If a node N advertises Prefix-SID SID-R for a prefix R that is attached to N and specifies CONTINUE as the operation to be performed by directly connected neighbors, then N MUST maintain the following FIB entry:
```
Incoming Active Segment: SID-R
Ingress Operation: NEXT
Egress interface: NULL
```

A remote node M MUST maintain the following FIB entry for any learned Prefix-SID SID-R attached to prefix R:

Incoming Active Segment: SID-R
Ingress Operation:
   If the next-hop of R is the originator of R
   and M has been instructed to remove the active segment: NEXT
   Else: CONTINUE
Egress interface: the interface(s) towards the next-hop along the
                  path computed using the algorithm advertised with
                  the SID toward prefix R.

As Prefix-SIDs are specific to a given algorithm, if traffic associated with an algorithm arrives at a node that does not support that algorithm, the traffic will be dropped as there will be no forwarding entry matching the incoming label.