Thursday, May 14, 2026

Cisco IOS XR Segment Routing with IS-IS Configuration Guide | MPLS SR Tutorial Part 3

Cisco IOS XR Segment Routing with IS-IS Configuration Guide Part 3

Complete Cisco IOS XR Segment Routing with IS-IS Configuration Guide - Part 3

In this third part of the Cisco IOS XR MPLS and Segment Routing series, we will migrate the MPLS core from OSPF to IS-IS while continuing to use Segment Routing MPLS.

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

📌 Important Lab Dependency

This lab assumes:

  • MPLS L3VPN topology from Part 1 already exists
  • Segment Routing using OSPF from Part 2 is already operational
  • SRGB is already configured on all routers

📚 Table of Contents

1. IS-IS Introduction

IS-IS stands for Intermediate System to Intermediate System.

IS-IS is one of the most widely used routing protocols in large service provider MPLS networks.

Many Tier-1 providers prefer IS-IS because of its scalability and flexibility.

🎯 Key Benefits of IS-IS

  • Excellent scalability
  • Protocol independence
  • Efficient flooding mechanism
  • Stable convergence
  • Ideal for MPLS and Segment Routing
  • Easy IPv6 integration
  • Supports Traffic Engineering

Shortest Path Formula

$$ Best\\ Path = Min(Cost_1 + Cost_2 + Cost_n) $$

IS-IS also uses the SPF algorithm for shortest path calculation.

2. Why IS-IS for Segment Routing?

Segment Routing integrates naturally with IS-IS.

IS-IS advertises Prefix-SIDs efficiently using TLVs.

Compared to OSPF:

  • IS-IS has simpler extensibility
  • IS-IS supports newer features faster
  • Many SPs standardize on IS-IS

IS-IS TLV Advertisement

$$ LSDB = \sum TLV_{Segments} $$

TLVs carry:

  • Prefix-SIDs
  • Adjacency-SIDs
  • SR capabilities
  • Traffic Engineering data

3. Migrating from OSPF to IS-IS

The first migration step is removing OSPF from all routers.

After that, IS-IS becomes the new IGP.

💡 Migration Strategy

  • Remove OSPF cleanly
  • Enable IS-IS Level-1
  • Configure wide metrics
  • Enable Segment Routing
  • Configure Prefix-SIDs
  • Enable SR Prefer

4. Task 1 - Remove OSPF

We now remove OSPF from all routers XR1 through XR5.

XR1 Remove OSPF

no router ospf 1

commit

XR2 Remove OSPF

no router ospf 1

commit

XR3 Remove OSPF

no router ospf 1

commit

XR4 Remove OSPF

no router ospf 1

commit

XR5 Remove OSPF

no router ospf 1

commit
What Happens When OSPF Is Removed?

Once OSPF is removed:

  • OSPF adjacencies disappear
  • Loopback reachability is lost temporarily
  • LDP sessions may drop
  • BGP VPNv4 sessions may flap

IS-IS restores the MPLS transport infrastructure.

5. Task 2 - Configure IS-IS

Now we configure IS-IS as the IGP between XR1 and XR5.

📌 Important Configuration Parameters

  • Area ID = 49.0000
  • Level-1 routers only
  • Metric-style wide enabled
  • IPv4 unicast enabled

XR1 IS-IS Configuration

router isis 1

 is-type level-1

 net 49.0000.1111.1111.1111.00

 address-family ipv4 unicast
  metric-style wide
 exit

 interface gig0/0/0/0
  address-family ipv4 unicast
 exit

 interface loopback0
  address-family ipv4 unicast
 exit
!

commit

XR2 IS-IS Configuration

router isis 1

 is-type level-1

 net 49.0000.2222.2222.2222.00

 address-family ipv4 unicast
  metric-style wide
 exit

 interface gig0/0/0/0
  address-family ipv4 unicast
 exit

 interface gig0/0/0/1
  address-family ipv4 unicast
 exit

 interface loopback0
  address-family ipv4 unicast
 exit
!

commit

XR3 IS-IS Configuration

router isis 1

 is-type level-1

 net 49.0000.3333.3333.3333.00

 address-family ipv4 unicast
  metric-style wide
 exit

 interface gig0/0/0/0
  address-family ipv4 unicast
 exit

 interface gig0/0/0/1
  address-family ipv4 unicast
 exit

 interface loopback0
  address-family ipv4 unicast
 exit
!

commit

XR4 IS-IS Configuration

router isis 1

 is-type level-1

 net 49.0000.4444.4444.4444.00

 address-family ipv4 unicast
  metric-style wide
 exit

 interface gig0/0/0/0
  address-family ipv4 unicast
 exit

 interface gig0/0/0/1
  address-family ipv4 unicast
 exit

 interface loopback0
  address-family ipv4 unicast
 exit
!

commit

XR5 IS-IS Configuration

router isis 1

 is-type level-1

 net 49.0000.5555.5555.5555.00

 address-family ipv4 unicast
  metric-style wide
 exit

 interface gig0/0/0/0
  address-family ipv4 unicast
 exit

 interface loopback0
  address-family ipv4 unicast
 exit
!

commit

6. Understanding IS-IS NET Address

NET stands for Network Entity Title.

The NET uniquely identifies IS-IS routers.

NET Structure

$$ NET = AreaID + SystemID + NSEL $$

Example:

$$ 49.0000.1111.1111.1111.00 $$

Breakdown:

  • 49.0000 = Area ID
  • 1111.1111.1111 = System ID
  • 00 = NSEL

💡 Why Metric-Style Wide?

Wide metrics are required for:

  • Segment Routing
  • Traffic Engineering
  • Modern IS-IS deployments
  • Extended metric support

7. Enable Segment Routing for IS-IS

Now we enable Segment Routing under IS-IS.

We also configure Prefix-SIDs and enable SR Prefer.

SR Prefix SID Formula

$$ NodeSID = SRGB + PrefixIndex $$

For XR5:

$$ 16000 + 5 = 16005 $$

XR1 IS-IS Segment Routing

router isis 1

 address-family ipv4 unicast
  segment-routing mpls sr-prefer
 exit

 interface loopback0
  address-family ipv4 unicast
   prefix-sid index 1
  exit
 exit
!

commit

XR2 IS-IS Segment Routing

router isis 1

 address-family ipv4 unicast
  segment-routing mpls sr-prefer
 exit

 interface loopback0
  address-family ipv4 unicast
   prefix-sid index 2
  exit
 exit
!

commit

XR3 IS-IS Segment Routing

router isis 1

 address-family ipv4 unicast
  segment-routing mpls sr-prefer
 exit

 interface loopback0
  address-family ipv4 unicast
   prefix-sid index 3
  exit
 exit
!

commit

XR4 IS-IS Segment Routing

router isis 1

 address-family ipv4 unicast
  segment-routing mpls sr-prefer
 exit

 interface loopback0
  address-family ipv4 unicast
   prefix-sid index 4
  exit
 exit
!

commit

XR5 IS-IS Segment Routing

router isis 1

 address-family ipv4 unicast
  segment-routing mpls sr-prefer
 exit

 interface loopback0
  address-family ipv4 unicast
   prefix-sid index 5
  exit
 exit
!

commit
What Does SR Prefer Do?

SR Prefer tells the router:

  • Prefer Segment Routing labels
  • Avoid traditional LDP labels when possible
  • Use deterministic Prefix-SIDs

This helps migrate networks away from LDP dependency.

8. Verification

Verify IS-IS Neighbors

show isis neighbors

Verify IS-IS Database

show isis database

Verify Segment Routing

show isis segment-routing

Verify MPLS Forwarding

show mpls forwarding

Traceroute Verification

traceroute 10.1.1.2 source 10.1.1.1

🎯 Expected Result

The SP Core should now use labels from:

$$ 16000 - 23999 $$

These are Segment Routing labels generated from the SRGB.

Expected MPLS Forwarding Output 
XR1#show mpls forwarding

16002  Pop    SR Pfx (10.1.1.2/32)

Label Computation Example

XR2:

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

Final label:

$$ 16000 + 2 = 16002 $$

9. Segment Routing Mathematics and Technical Deep Dive

SPF Calculation

$$ SPF = Min(\sum LinkCost) $$

Label Stack Structure

$$ Packet = NodeSID + ServiceSID + Payload $$

Traffic Engineering Path

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

IS-IS Scalability

$$ IS\\text{-}IS_{Scale} > OSPF_{Scale} $$

IS-IS generally scales better in very large SP networks.

Deterministic SID Mapping

$$ SID(Label)=SRGB+Index $$

10. Troubleshooting

Issue Cause Solution
No IS-IS adjacency Interface not enabled Enable IS-IS under interface
No Prefix SID Prefix SID missing Configure prefix-sid index
LDP labels still visible SR Prefer missing Enable sr-prefer
No reachability NET mismatch Verify area ID and System-ID

Useful Verification Commands

show isis neighbors

show isis database

show route

show mpls forwarding

show segment-routing mpls forwarding

show cef

🎯 Final Conclusion

In this third part of the Cisco IOS XR Segment Routing series, we successfully migrated the MPLS core from OSPF to IS-IS while maintaining Segment Routing MPLS forwarding.

We configured:

  • IS-IS Level-1 routing
  • Wide metrics
  • NET addresses
  • Segment Routing under IS-IS
  • Prefix-SIDs
  • SR Prefer
  • MPLS SR forwarding

We also learned:

  • How IS-IS integrates with Segment Routing
  • Why IS-IS is preferred in SP networks
  • How Prefix-SIDs generate deterministic labels
  • How SR forwarding replaces traditional LDP labels
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