SR-MPLS Inter AS

The inter-AS option C was set up, and this year, we tested another round that routing could be optimized by deploying anycast SID within the BGP-SR structure for inter-domain connectivity. An anycast IP address was advertised by several ASBRs, with the same prefix SID associated, thus ensuring that packets were automatically routed towards the ASBR that was topologically closest. This allows the attaching of a service label, a node label, and an anycast label by the label stack (set by egress PEs), allowing end-to-end SR-MPLS operation with predictable label values and fast convergence. Option-C was used to distinguish inter-AS transport from service VPN routes, where the ASBRs acted as stitching points between AS1 and AS2, functioning as inline route reflectors when required. 

The test demonstrates interoperability for the following:

• BGP: RFC 8669, SR Prefix Segment Identifier Extensions for BGP
• Label-Index TLV
• Optionally, Originator SRGB
• Next-Hop set to Anycast IP address
• BGP-LU label set to BGP-SID (SRGB+Index)
• ISIS: Advertise in L-ISIS Anycast IP + Prefix-SID
• MPLS Data plane: Labeled Unicast

Control plane updates used BGP-LU to advertise the prefix SID per RFC 8669 with optional SRGB information. IS-IS in the IGP domain was meant to advertise loopback addresses and prefix SIDs for every router, including anycast address. In the failure event of ASBR, traffic is redirected to another ASBR with the help of the IGP’s fast‐reroute path instead of waiting for BGP to reconverge. This approach showed reliable interoperability of anycast routing for inter‐domain SR‐MPLS. It confirmed that a single anycast SID could serve multiple ASBRs, letting packets automatically reroute to an alternate boundary node during failures.

The value of the test for Operators:

• Fast Reroute (TI-LFA) Across Domain Boundaries: Because multiple ASBRs advertise the same Anycast next hop, the network can quickly redirect packets to an alternate ASBR when one fails.
• End-to-end SR-MPLS with Predictable Labels: The label assignment would be more deterministic if prefix SIDs and a known SRGB were used.
• Scalability with BGP Option-C: Separating transport labels from service labels keeps routing tables more manageable and less complex.
• ASBR “Stitching” for Multiple Domains: A single stitching ASBR can connect multiple domains, and additional ASBRs provide redundancy for important paths.
  Test Procedure and Observations
• Labeled IS-IS: Verify database and SR TLVs (Loopbacks and Prefix SIDs).
• L3VPN, as Usual, with Option-C for Inter-AS: The route reflectors are peering across the AS boundary.
• ASBR acts as a stitching node, as inline RR for BGP-LU + Prefix-SIDs: -Data plane stitching, -Control plane: Prefix-SID + SRGB
• ISIS metric on the P node forces traffic flow to the right ASBR
• Failure Scenarios: In the event of an ASBR failure, traffic was fast-rerouted to the next available ASBR using TI-LFA (long before BGP convergence kicked in).