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Label Distribution Protocol
Note:
10. Router Rx, which owns prefix X, is upgraded to support SR. Rx sends a prefix
11. The user then cleans up the mapping server and removes the mapping entry for
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4. The IGP in R1 attempts to resolve in its routing table the next hop of prefix X over
the interface to R2. R1 detects that R2 did not advertise support of SR and
therefore the SID resolution for prefix X in the routing table fails.
5. The IGP in R1 then attempts to resolve the prefix SID of prefix X in the TTM
because it detects that it is configured for SR-to-LDP stitching. R1 finds an LDP
tunnel to prefix X in the TTM, instructs the SR module to program an SR ILM with
ingress label L3, and points it to the LDP tunnel endpoint, thus stitching ingress
label L3 to egress label L2.
• The ILMs for LDP and SR are both pointing to the same LDP tunnel, one via NHLFE
and one via the tunnel endpoint.
• No SR tunnel to destination prefix X should be programmed in the TTM following the
resolution of the prefix SID of prefix X in the TTM.
• If the IGP is not able to resolve the SID resolution for prefix X in step
trap is generated for the prefix SID resolution failure. An existing trap for the prefix SID
resolution failure is enhanced to state whether the prefix SID that failed the resolution
attempts was part of a mapping server TLV or an IP reachability TLV.
6. The user enables segment routing on R2.
7. The IGP in R1 discovers that R2 supports SR.
Because R1 still has a prefix SID for prefix X from the mapping server R5, it
maintains the stitching of the SR ILM for prefix X to the LDP FEC.
8. The user disables the LDP interface between R1 and R2 in both directions. This
causes the LDP FEC ILM and NHLFE for prefix X to be removed in R1 and
triggers the re-evaluation of the SIDs.
9. R1 first attempts the resolution in the routing table. Because the next hop for
prefix X supports SR, the IGP instructs the SR module to program an NHLFE for
the prefix SID of prefix X with egress label L4 and with an outgoing interface to
R2. R1 creates an SR tunnel in the TTM for destination prefix X. R1 also
changes the SR ILM with ingress label L3 to point to the SR NHLFE with egress
label L4.
Router R2 now becomes the SR-LDP stitching router.
SID sub-TLV to R1 in an IS-IS IP reachability TLV for prefix X. The SID
information may or may not be the same as the information received from the
mapping server R5. If the SID information is not the same, the IGP in R1
chooses the prefix SID originated by Rx and updates the SR ILM and NHLFE
with the appropriate labels.
prefix X, which is then withdrawn by IS-IS.
Use subject to Terms available at: www.nokia.com
© 2022 Nokia.
MPLS Guide
4
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