Dual-Stack SR-MPLS

Key Takeaways

• IPv6 node SIDs require /128 loopback prefixes on Arista EOS and Cisco XR
• Cisco IOS XE does not support IPv6 SIDs in SR‑MPLS
• FRRouting, Junos, Nokia SR Linux assign IPv6 SIDs automatically
• Netlab automates dual‑stack addressing and IS‑IS SR‑TLV generation
• Mismatched prefix lengths lead to silent IS‑IS advertisement failures

Pulse Analysis

Segment Routing over MPLS has become a cornerstone for modern traffic engineering, yet many operators still run IPv4‑only deployments. Introducing dual‑stack SR‑MPLS allows the same node segment identifiers to steer both IPv4 and IPv6 traffic, simplifying policy enforcement and reducing the need for separate control planes. By configuring loopback interfaces with IPv4 /32 and IPv6 /128 prefixes, the lab shows how IS‑IS can distribute SR Prefix‑SID TLVs for both families, enabling seamless label‑switched paths across mixed‑protocol networks.

The practical challenge lies in vendor‑specific handling of IPv6 prefix lengths. While FRRouting, Junos, and Nokia SR Linux automatically advertise IPv6 SIDs, Arista EOS and Cisco IOS XR only expose them when the loopback is a /128, and Cisco IOS XE outright lacks IPv6 SID support. This discrepancy can trap engineers in hours of silent troubleshooting, as the IS‑IS database may appear correct while the MPLS forwarding plane lacks the expected labels. Understanding these nuances is essential for network architects planning IPv6 rollouts, ensuring that prefix design aligns with each platform’s requirements.

For teams ready to experiment, the SR‑MPLS workshop repository provides a turnkey netlab environment that can be launched in a GitHub Codespace or an Arista cEOS container. After a simple "netlab up" command, the three‑router topology is up and running, complete with IS‑IS adjacency, SR‑TLVs, and MPLS label tables. This hands‑on approach not only validates dual‑stack configurations but also serves as a reference for production deployments, helping operators avoid common pitfalls and accelerate the transition to a fully IPv6‑enabled, segment‑routed network.