SR-MPLS Inter AS

Inter‐domain service continuity remains a key challenge for operators seeking to extend SR‐MPLS–based VPNs across multiple autonomous systems. In this test, we evaluated an Inter‐AS Option C solution enhanced with Anycast SIDs, allowing multiple ASBRs to advertise the same IP prefix. The goal was to demonstrate how our design can optimize traffic steering by directing packets to the topologically nearest ASBR and how the approach ensures rapid failover.
From the operator’s perspective, this design offers several practical advantages. In the event of an ASBR failure, Fast Reroute (TI‐LFA) quickly redirects traffic to another ASBR without waiting for a full BGP re-convergence. Option C reduces routing complexity and promotes scalability by separating transport labels from service labels. Also, anycast addressing simplifies domain stitching by letting multiple boundary nodes appear as a single entry point, improving load balancing and resilience in complicated, multi‐domain deployments.
This year’s test cycle focused on enhancing inter-domain routing in the BGP-SR structure by introducing an anycast SID, following the vendors' earlier deployment of the inter-AS option C.
Several ASBRs advertised an anycast IP address with the same prefix SID associated, thus ensuring that DUTs routed packets automatically towards the ASBR that was topologically closest. Attaching a service label, a node label, and an anycast label (set by egress PEs) enables end-to-end SR-MPLS operation with predictable label values, efficient usage, 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 traffic generator was connected via a switch, enabling simultaneous communication with all PE devices.

The test validated the interoperability of key BGP features, including the Segment Routing Prefix SID extensions defined in RFC 8669, Label-Index TLV, and optionally, the Originator SRGB. The next hop was configured with an anycast IP address, and BGP-LU labels were assigned using the BGP-SID format (SRGB + Index). In the IGP domain, L-ISIS advertised the anycast IP and the associated Prefix-SID. The data plane operated over MPLS using labeled unicast forwarding.

Control plane updates used BGP-LU to advertise the prefix SID per RFC 8669 with optional SRGB information. We used ISIS in the IGP domain to advertise loopback addresses and prefix SIDs for every router, including anycast addresses. In the event of an ASBR failure, 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.
 This test demonstrated several operational advantages for service providers. First, Topology Independent Loop-Free Alternate (TI-LFA) enabled fast reroute capabilities across domain boundaries, ensuring packets could be redirected seamlessly to alternate ASBRs advertising the same anycast next hop. It also validated that SR-MPLS label assignment can be predictable through consistent use of prefix-SIDs and a known SRGB, improving operational determinism. Additionally, using BGP Option-C to separate transport and service labels contributes to simpler and more scalable routing table management. Finally, stitching ASBRs effectively connected multiple domains, with added ASBRs enhancing path redundancy and fault tolerance.
 The testing process involved verifying SR TLVs and loopback/prefix SID consistency under labeled ISIS. L3VPN was validated using Option-C peering across AS boundaries. The ASBR functioned as a stitching node and an inline route reflector, handling BGP-LU and prefix-SID dissemination. The data plane relied on prefix-SID + SRGB mapping. Traffic flow was steered through the correct ASBR using ISIS metric tuning. Rerouting occurred via TI-LFA mechanisms in failure scenarios, ensuring uninterrupted delivery before BGP reconvergence.