Segment Routing with IS-IS: Practical Migration and Configuration

Migration from LDP/RSVP-TE to Segment Routing (SR-MPLS or SRv6). A simplified MPLS control plane, programmable Traffic Engineering, and greater scalability. Supported by Juniper (Junos MX, PTX), Cisco IOS-XR (ASR 9000, NCS), and Nokia 7750. 2026 SR migration guide.

Why SR

• No need for LDP or RSVP-TE (resource-intensive protocols)
• Labels distributed by the IGP (OSPF or IS-IS extensions)
• Source routing: the headend adds a label stack (segments)
• Simple and scalable
• Programmable TE

SR Concepts

• Prefix SID / Node SID: identifies a router
• Adjacency SID: identifies a link
• SRGB (Segment Routing Global Block): default label range 16000-23999
• Binding SID: composite SID for an SR Policy

SR-MPLS vs. SRv6

• SR-MPLS: MPLS data plane (labels), IPv4/IPv6
• SRv6: IPv6 data plane (IPv6 extension header), more flexible but newer
• Adoption in 2026: SR-MPLS is mainstream among ISPs; SRv6 is used by 5G operators

LDP → SR Migration

Phase 1: Coexistence

• Enable SR on the IGP (IS-IS or OSPF)
• LDP remains active in parallel
• Verify label consistency

Phase 2: SR as Primary

• Set SR preference using mpls traffic-engineering sr-preference
• LDP remains the backup

Phase 3: Decommission LDP

• Graceful removal
• Monitor BGP/LDP sessions

Juniper SR-ISIS Configuration

• set protocols isis source-packet-routing srgb start-label 16000 index-range 8000
• set protocols isis source-packet-routing node-segment ipv4-index 100
• set interfaces ge-0/0/0 unit 0 family iso

Cisco IOS-XR Configuration

• segment-routing
• global-block 16000 23999
• router isis 1
• address-family ipv4 unicast
• metric-style wide
• segment-routing mpls

SR-TE (Traffic Engineering)

• SR Policy: ordered list of segments to traverse
• Centralized controller (PCE)
• Examples: Cisco Crosswork Network Controller, Juniper NorthStar, Nokia WaveSuite

SR Fast ReRoute (FRR)

• TI-LFA (Topology-Independent Loop-Free Alternate)
• Backup path precomputed by the IGP
• Convergence <50ms
• Replaces RSVP-TE FRR

Tools

• Cisco Crosswork: SR orchestration
• Juniper Paragon (formerly NorthStar): SR-TE
• Nokia NSP (Network Services Platform)
• Telemetry: streaming gNMI for SR monitoring

Order from OPTINOC

Turnkey SR-MPLS migration. Juniper MX, Cisco ASR, Nokia 7750. Training + POC + programmable TE. ISP quote within one week.