Segment Routing Deployment – Control and Data Plane explains how to deploy SR in service provider network. The shared example was very good in case of green field deployment. But most of the service providers or large enterprise have already deployed LDP in the network. So it makes sense to have new networks deployed with SR-MPLS and existing will run on the LDP only. In this design, the SR and LDP interworking is required.

Technical Deep Dive Of Segment Routing Control Plane – ISIS
Segment Routing makes use of the Segment Routing Global Block (SRGB). The use of the SRGB allows SR to co-exist with any other control plane protocol installing forwarding entries in the MPLS data plane.

LDP to SR Behavior: An SR node having LDP neighbors MUST create LDP bindings for each Prefix-SID learned in the SR domain by treating SR learned labels as if they were learned through an LDP neighbor. In addition, for each FEC, the SR node stitches the incoming LDP label to the outgoing SR label.

SR to LDP: SR to LDP interworking requires a Segment Routing Mapping Server (SRMS). Each SR capable router installs in the MPLS data plane Node-SIDs learned from the SRMS exactly like if these SIDs had been advertised by the nodes themselves. An SR node having LDP neighbors MUST stitch the incoming SR label (whose SID is advertised by the SRMS) to the outgoing LDP label.

In this post, I will be covering how to configure segment routing mapping server (SRMS) on S1 node which will announce prefix-sid for the LDP neigbors (P6-6.6.6.6/32 and PE7-7.7.7.7/32)

Currently Segment Routing Mapping Server(SRMS) is not configured and check how the control plane mapping is working in SR-LDP network. P6,PE7 and RR-P8 are LDP nodes.

[LDP to SR Path]
Below traceroute output explicitly shows that PE7 has labelled path available for L4

RP/0/0/CPU0:PE7#traceroute 4.4.4.4 source 7.7.7.7
1 6.7.0.6 [MPLS: Label 24020 Exp 0] 19 msec 19 msec 19 msec
2 5.6.0.5 [MPLS: Label 24019 Exp 0] 19 msec 19 msec 9 msec
3 1.5.0.1 [MPLS: Label 16004 Exp 0] 9 msec 9 msec 19 msec
4 1.4.0.4 9 msec * 19 msec
[SR to LDP Path]
Below traceroute output explicitly shows that L4 has no SR bindings avaiable for PE7.

RP/0/0/CPU0:L4#traceroute sr-mpls 7.7.7.7/32 source 4.4.4.4
0 4.4.4.4 MRU 0 [No Label]
Q 1 *
RP/0/0/CPU0:L4#
Below output shows that L4 doesn’t have prefix-sid (labels) avaiable for PE7.

RP/0/0/CPU0:L4# show isis segment-routing label table

IS-IS lab IS Label Table
Label Prefix/Interface
———- —————-
16001 1.1.1.1/32
16002 2.2.2.2/32
16003 3.3.3.3/32
16004 Loopback0
16005 5.5.5.5/32