Complete Cisco IOS XR SR-LDP Mapping Server Configuration Guide | Segment Routing Interoperability Part 4

Complete Cisco IOS XR SR-LDP Mapping Server Configuration Guide - Part 4

In this fourth part of the Cisco IOS XR MPLS and Segment Routing series, we will configure SR-LDP interoperability using an SR Mapping Server.

This lab demonstrates one of the most important migration strategies used by service providers when moving from traditional MPLS LDP networks toward Segment Routing.

📌 Important

This lab is built on the topology and configurations from:

• Part 1 - MPLS L3VPN

📚 Table of Contents

• 1. SR-LDP Interoperability Introduction
• 2. Why Mapping Server is Required
• 3. Hybrid MPLS Topology
• 4. Task 1 - Remove SR from XR1 & XR2
• 5. Task 2 - Remove LDP from XR4 & XR5
• 6. Understanding SR Mapping Server
• 7. Task 3 - Configure SR Mapping Server
• 8. Verification
• 9. Segment Routing Mathematics
• 10. Troubleshooting
• 11. Related Articles

1. SR-LDP Interoperability Introduction

Most service providers cannot migrate an entire MPLS network from LDP to Segment Routing instantly.

Real-world migration occurs gradually.

Some routers continue using:

• LDP
• Traditional MPLS

While newer routers use:

• Segment Routing
• Prefix-SIDs
• SR MPLS forwarding

🎯 Goal of This Lab

Create interoperability between:

• LDP routers
• Segment Routing routers

using:

• SR Mapping Server

Hybrid MPLS Formula

$$ MPLS_{Hybrid}=LDP+SegmentRouting $$

2. Why SR Mapping Server Is Required

LDP routers do not understand Prefix-SIDs.

Segment Routing routers may not understand LDP labels after migration.

The SR Mapping Server solves this problem.

💡 SR Mapping Server Responsibilities

• Maps prefixes to Prefix-SIDs
• Advertises SID mappings into IS-IS
• Provides interoperability between SR and LDP domains
• Supports gradual migration

Prefix Mapping Formula

$$ Prefix \rightarrow SID $$

Example:

$$ 10.1.1.1/32 \rightarrow SID100 $$

3. Hybrid MPLS Topology

XR1 ---- XR2 ---- XR3 ---- XR4 ---- XR5

LDP      LDP      Hybrid     SR      SR

After this migration:

• XR1 and XR2 use LDP
• XR4 and XR5 use Segment Routing
• XR3 becomes the interoperability point

🎯 XR3 Becomes:

• LDP Router
• SR Router
• SR Mapping Server

4. Task 1 - Remove Segment Routing from XR1 & XR2

We first remove Segment Routing from XR1 and XR2.

These routers will revert back to traditional LDP.

XR1 Remove Segment Routing

router isis 1

 address-family ipv4 unicast
  no segment-routing mpls sr-prefer
 exit

 interface loopback0
  address-family ipv4 unicast
   no prefix-sid index 1
  exit
 exit
!

commit

XR2 Remove Segment Routing

router isis 1

 address-family ipv4 unicast
  no segment-routing mpls sr-prefer
 exit

 interface loopback0
  address-family ipv4 unicast
   no prefix-sid index 2
  exit
 exit
!

commit

What Happens After Removing SR?

After removing Segment Routing:

• Prefix-SIDs disappear
• Routers stop advertising SR labels
• LDP becomes primary again
• MPLS forwarding uses LDP labels

LDP Fallback Logic

$$ If\\ SR=Disabled $$ $$ Then\\ MPLS=LDP $$

5. Task 2 - Remove LDP from XR4 & XR5

Now we remove LDP from XR4 and XR5.

These routers will continue using Segment Routing only.

XR3 Remove LDP on XR3-XR4 Link

mpls ldp

 no interface gig0/0/0/1
!

commit

XR4 Remove LDP

no mpls ldp

commit

XR5 Remove LDP

no mpls ldp

commit

🎯 Current MPLS State

Router	Forwarding Method
XR1	LDP
XR2	LDP
XR3	Hybrid SR + LDP
XR4	Segment Routing
XR5	Segment Routing

6. Understanding SR Mapping Server

The Mapping Server advertises SID mappings on behalf of routers that do not support Segment Routing.

XR1 and XR2 no longer advertise Prefix-SIDs.

XR3 will now advertise SID mappings for them.

Mapping Server Advertisement

$$ RouterWithoutSR \rightarrow MappingServer \rightarrow PrefixSID $$

💡 Important Concept

The Mapping Server:

• Does NOT allocate labels dynamically
• Statically maps prefixes to SIDs
• Advertises mappings using IS-IS

7. Task 3 - Configure SR Mapping Server

Now we configure XR3 as the SR Mapping Server.

XR3 will advertise SID mappings for XR1 and XR2 loopbacks.

XR3 SR Mapping Server Configuration

segment-routing

 mapping-server

  prefix-sid-map address-family ipv4

   10.1.1.1/32 100 range 3
!

router isis 1

 address-family ipv4 unicast

  segment-routing prefix-sid-map advertise-local
!

commit

Deep Explanation of Prefix-SID Map

This command:

10.1.1.1/32 100 range 3

means:

• Start prefix = 10.1.1.1
• Base SID = 100
• Range = 3 prefixes

Therefore:

Prefix	SID
10.1.1.1	100
10.1.1.2	101
10.1.1.3	102

Prefix SID Mapping Formula

$$ SID = BaseSID + Offset $$

Example:

$$ 101 = 100 + 1 $$

8. Verification

Verify Mapping Server

show segment-routing mapping-server prefix-sid-map

Verify MPLS Forwarding

show mpls forwarding

Verify IS-IS Segment Routing

show isis segment-routing

Traceroute Verification

traceroute 10.1.1.5 source 10.1.1.1

🎯 Expected Result

You should notice:

• LDP labels before XR3
• Segment Routing labels after XR3

This proves SR-LDP interoperability is working successfully.

Expected Label Behavior

Topology Segment	Expected Labels
XR1 → XR2	24XXX LDP Labels
XR3 → XR5	16XXX SR Labels

Hybrid MPLS Label Flow

$$ LDP \rightarrow MappingServer \rightarrow SegmentRouting $$

9. Segment Routing Mathematics and Deep Technical Analysis

SRGB Formula

$$ Label = SRGB + SID $$

Example Calculation

$$ 16000 + 101 = 16101 $$

Hybrid Forwarding Logic

$$ Forwarding = LDP + SR $$

Migration Formula

$$ Migration_{Safe}=IncrementalTransition $$

Segment Routing Scalability

$$ Complexity_{SR} < Complexity_{LDP} $$

10. Troubleshooting

Issue	Cause	Solution
No SR labels	Prefix SID mapping missing	Verify mapping-server config
No LDP labels	LDP disabled accidentally	Verify LDP interfaces
No traceroute continuity	Hybrid forwarding broken	Verify XR3 interoperability
No SR mapping advertisement	advertise-local missing	Enable advertise-local

Useful Verification Commands

show segment-routing mapping-server prefix-sid-map

show isis segment-routing

show mpls forwarding

show cef

show route

11. Related Articles

• Cisco IOS XR MPLS L3VPN Configuration Guide Part 1
• Cisco IOS XR Segment Routing with OSPF Configuration Guide Part 2
• Cisco IOS XR Segment Routing with IS-IS Configuration Guide Part 3

🎯 Final Conclusion

In this fourth part of the Cisco IOS XR Segment Routing series, we successfully configured SR-LDP interoperability using an SR Mapping Server.

We learned:

• How to migrate gradually from LDP to SR
• How SR Mapping Server works
• How Prefix-SID mappings are advertised
• How hybrid MPLS forwarding operates
• How labels transition between LDP and SR domains

We configured:

• SR removal on XR1/XR2
• LDP removal on XR4/XR5
• Hybrid forwarding on XR3
• SR Mapping Server
• Prefix-SID mappings