Unified MPLS resolves legacy challenges such as scaling MPLS to support tens of thousands of end nodes, which provides the required MPLS functionality on cost-effective platforms, and the complexity of technologies like traffic engineering fast reroute (TE-FRR) to meet transport SLAs.
Large mobile service providers scales MPLS infrastructure using RFC 3107 hierarchical LSPs based architecture. The other areas PEs routes received as BGP labeled routes instead of distributing in IGP. ABR’s work as inline RR with next-hop-self so that every IGP domain can reach to anywhere in the netowork by adding label of it’s inline RR.
When SR will deploy along with the same network in hierarchy, question comes in mind how the prefix-SIDs of these labelled BGP routes get distributed in SR domain. Where do I place my segment routing mapping server (SRMS)?
The answer to the question is really don’t need to install or create any kind of SRMS functionality in the SR domain. SRMS functionality is only required when we redistribute LDP the routes from LDP to SR domain or vice-versa. But in case of Unified MPLS, PE’s loopbacks of other domains will still learn via labelled BGP on ABR’s. ABR’s are inline route reflectors. ABR’s will distribute these labeled prefixes in SR domain.
On ingress SR PE, destination VPNv4 prefix will be learnt by vpnv4 route. The next hop of this VPNv4 prefix will be the ABR (inline RR). So ingress PE will add the VPNv4 label and SR label (prefix-SID) of ABR. Once packet will reach to SR-ABR, SR-ABR will do the stitching of SR to LDP and forward the packet by swaping it’s label.