Thursday, May 14, 2026

Complete Cisco IOS XR Segment Routing with OSPF Configuration Guide | SRGB, Prefix-SID & SR-Prefer

Cisco IOS XR Segment Routing with OSPF Configuration Guide Part 2

Complete Cisco IOS XR Segment Routing with OSPF Configuration Guide - Part 2

In this second part of the Cisco IOS XR MPLS and Segment Routing series, we will migrate the existing MPLS L3VPN topology toward Segment Routing using OSPF.

This lab is built directly on the MPLS L3VPN topology configured in Part 1.

📌 Important

Before starting this lab, complete Part 1 because all configurations in this article depend on the MPLS L3VPN core already being operational.

📚 Table of Contents

1. Segment Routing Introduction

Segment Routing is the modern evolution of MPLS networks.

Traditional MPLS depends heavily on protocols such as:

  • LDP
  • RSVP-TE
  • Complex signaling protocols

Segment Routing simplifies MPLS architecture by removing many signaling protocols and replacing them with source-routed instructions called Segments.

🎯 Main Benefits of Segment Routing

  • Simpler MPLS architecture
  • Reduced control-plane complexity
  • Faster convergence
  • Traffic engineering without RSVP
  • Scalable modern SP architecture
  • Excellent SDN integration
  • Supports automation and programmability

Segment Routing Path Formula

$$ Path = SID_1 + SID_2 + SID_3 + \dots + SID_n $$

Each SID represents:

  • A node
  • An adjacency
  • A service
  • A specific instruction

2. Why Segment Routing Over LDP?

LDP dynamically allocates labels independently.

Segment Routing creates deterministic label allocation.

With SR:

  • No targeted LDP sessions
  • No RSVP signaling
  • No label exchange complexity
  • Labels become topology-aware

LDP vs SR Comparison

$$ LDP = Dynamic\\ Label\\ Allocation $$
$$ SR = Deterministic\\ Prefix\\ SID $$

3. Existing MPLS Topology


CE1 ---- XR1 ---- XR2 ---- XR3 ---- XR4 ---- XR5 ---- CE2

         PE        P        RR       P        PE

The topology from Part 1 already includes:

  • OSPF Core
  • MPLS LDP
  • MP-BGP VPNv4
  • VRFs
  • MPLS L3VPN services

Now we are enabling Segment Routing on top of the same infrastructure.

4. Understanding SRGB

SRGB stands for Segment Routing Global Block.

The SRGB defines the global label range used by Segment Routing.

SRGB Formula

$$ PrefixSID\\ Label = SRGB\\ Base + PrefixSID\\ Index $$

Example:

$$ 16000 + 5 = 16005 $$

Therefore:

$$ PrefixSID\\ 5 = Label\\ 16005 $$

💡 Important Concept

Every router in the SR domain should ideally use the same SRGB range.

In this lab:

$$ SRGB = 16000 - 23999 $$

5. Task 1 - Configure Segment Routing Global Block

Now we configure SRGB on all routers XR1 through XR5.

XR1 SRGB Configuration

segment-routing global-block 16000 23999

commit

XR2 SRGB Configuration

segment-routing global-block 16000 23999

commit

XR3 SRGB Configuration

segment-routing global-block 16000 23999

commit

XR4 SRGB Configuration

segment-routing global-block 16000 23999

commit

XR5 SRGB Configuration

segment-routing global-block 16000 23999

commit
Deep Explanation of SRGB

The SRGB defines the local label allocation space reserved for Segment Routing.

Labels within this range become globally meaningful across the SR domain.

For example:

Prefix SID Computed Label
1 16001
2 16002
3 16003
4 16004
5 16005

6. Understanding Prefix-SID

A Prefix-SID identifies a network prefix using a globally known MPLS label.

Instead of dynamically assigning labels using LDP, routers advertise Prefix-SIDs through OSPF.

Prefix SID Formula

$$ SID\\ Label = SRGB\\ Base + SID\\ Index $$

For XR3:

$$ 16000 + 3 = 16003 $$

XR3 loopback uses label:

$$ 16003 $$

7. Task 2 - Configure OSPF Segment Routing

Now we enable Segment Routing under OSPF.

We also assign Prefix-SID indexes using Loopback0 interfaces.

🎯 Prefix SID Mapping

  • XR1 = SID 1
  • XR2 = SID 2
  • XR3 = SID 3
  • XR4 = SID 4
  • XR5 = SID 5

XR1 Segment Routing OSPF

router ospf 1

 segment-routing mpls

 area 0

  interface loopback0
   prefix-sid index 1
!

commit

XR2 Segment Routing OSPF

router ospf 1

 segment-routing mpls

 area 0

  interface loopback0
   prefix-sid index 2
!

commit

XR3 Segment Routing OSPF

router ospf 1

 segment-routing mpls

 area 0

  interface loopback0
   prefix-sid index 3
!

commit

XR4 Segment Routing OSPF

router ospf 1

 segment-routing mpls

 area 0

  interface loopback0
   prefix-sid index 4
!

commit

XR5 Segment Routing OSPF

router ospf 1

 segment-routing mpls

 area 0

  interface loopback0
   prefix-sid index 5
!

commit
What Happens Internally?

Once Segment Routing is enabled:

  • OSPF advertises Prefix-SIDs inside LSAs
  • Routers compute SR labels automatically
  • MPLS forwarding uses SR labels
  • LDP still remains active initially

8. Verification Before SR Prefer

Before enabling SR prefer, the network still prefers traditional LDP labels.

Traceroute Verification

traceroute 10.1.1.2 source 10.1.1.1

Expected Result

The MPLS core should still use:

📌 Non-SR Label Block

The labels should be from:

$$ 24XXX $$

This indicates LDP labels are still preferred.

Sample MPLS Forwarding Output 
XR1#show mpls forwarding

24001  Pop    SR Pfx (10.1.1.2/32)

LDP Label Logic

$$ LDP\\ Labels \in 24000-24999 $$

These are dynamically allocated labels.

9. Task 3 - Configure SR Prefer

Now we instruct OSPF to prefer Segment Routing labels instead of LDP labels.

🎯 Purpose of SR Prefer

  • Prefer Prefix-SIDs over LDP labels
  • Migrate toward SR forwarding
  • Keep LDP as fallback if needed
  • Simplify MPLS operations

XR1 SR Prefer

router ospf 1

 segment-routing sr-prefer
!

commit

XR2 SR Prefer

router ospf 1

 segment-routing sr-prefer
!

commit

XR3 SR Prefer

router ospf 1

 segment-routing sr-prefer
!

commit

XR4 SR Prefer

router ospf 1

 segment-routing sr-prefer
!

commit

XR5 SR Prefer

router ospf 1

 segment-routing sr-prefer
!

commit

10. Verification After SR Prefer

Now verify the forwarding behavior again.

Traceroute Verification

traceroute 10.1.1.2 source 10.1.1.1

🎯 Expected Result

Now the MPLS core should use:

$$ 16000 - 23999 $$

Specifically:

$$ 16002 $$

for XR2 loopback.

Expected MPLS Forwarding Output 
XR1#show mpls forwarding

16002  Pop    SR Pfx (10.1.1.2/32)

SR Label Computation

For XR2:

$$ SRGB = 16000 $$ $$ PrefixSID = 2 $$

Final label:

$$ 16000 + 2 = 16002 $$

11. Segment Routing Mathematics and Deep Technical Concepts

Node SID Formula

$$ NodeSID = SRGB + PrefixIndex $$

Label Stack Example

$$ Packet = TransportSID + ServiceSID + Payload $$

Traffic Engineering Path

$$ Path = SID_1 \rightarrow SID_2 \rightarrow SID_3 $$

Shortest Path First Calculation

$$ SPF = Min(Cost_1 + Cost_2 + Cost_n) $$

Segment Routing Scalability

$$ Complexity_{SR} < Complexity_{LDP+RSVP} $$

Segment Routing significantly reduces operational complexity.

12. Troubleshooting Segment Routing

Issue Cause Solution
No Prefix SID OSPF SR disabled Enable segment-routing mpls
Wrong labels Incorrect SRGB Verify SRGB consistency
LDP labels still used SR prefer missing Enable sr-prefer
No SR forwarding Missing Prefix SID Configure prefix-sid index

Useful Verification Commands

show ospf segment-routing

show segment-routing mpls forwarding

show mpls forwarding

show route

show cef

🎯 Final Conclusion

In this second part of the MPLS and Segment Routing series, we successfully migrated the existing MPLS topology toward Segment Routing using OSPF.

We configured:

  • SRGB
  • Prefix-SIDs
  • OSPF Segment Routing
  • SR label computation
  • SR preferred forwarding
  • MPLS SR verification

We also learned the difference between:

  • LDP labels
  • Segment Routing labels
  • Dynamic vs deterministic label allocation
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