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Generation old networks are operated using LDP based MPLS and it is not possible to migrate to Segment Routing based network over-night or some part of the network may not support SR due to hardware or software limitations. For that reason , existing LDP and SR will co-exist for some time and there has to be interworking with each other. SR and LDP interworking helps in migration process.
Seamless interworking between LDP and SR is achieved by Mapping server functionality. Mapping server basically map LDP label to Prefix SID and sits at the border of two networks.
Lets understand LDP-SR interworking with example . Refer figure 1 below,
Nodes on left side are LDP enabled nodes and on right side are SR enabled nodes. Node S3 on the border of two networks is enabled for both LDP and SR and acts as mapping server. Node N1,N2 and N3 are LDP nodes so they use dynamic label range above 24000 and nodes S1 and S2 are SR nodes so they use SRGB range for prefix segment id (SID) . Node S3 is enabled for both LDP and SR so it uses both. Lets see how LDP Node N1 can reach SR enabled node S1 over label switched path.Here are the steps how forwarding entries are stitches to have end-to-end LSP ,
Step 1 – Node S1 advertise its Prefix SID 16302 for its loopback address , node S2 and S3 installs the forwarding entry for prefix SID 16302 to Node S1.
Step 2 – LDP on Node S3 dynamically allocates label 90001 for the loopback address of node S1 and advertise this binding information to its LDP neighbours. So , border Node S3 (mapping server ) keeps mapping between LDP label and Prefix SID for the loopback address of node S1.
Step 3 – LDP on Node N3 install local label 90010 with out going label 90001 . Similarly, all other nodes install LDP local label and out going label for the loopback prefix address of Node S1.
Step 4 – Node S3 automatically connects LDP LSP to the prefix segment to reach Node S1 and provides seamless label switch path from node N1 to node S1. This functionality is called mapping server functionality.
Interworking applies to only labeled packets . For unlabeled IP packets with destination to Node S1 above, node S3 imposes prefix SID 16302.
In Summary, interworking is automatic and seamless and no specific configuration is required except mapping server functionality. This is the easiest way that LDP and SR based networks can not only co-exist but also communicate with each other to provide end-to-end LSP path. I hope this article is useful in understanding LDP/SR interworking concept.