Thursday, May 14, 2026

Complete Cisco IOS XR MPLS L3VPN Configuration Guide Part 1 | OSPF, ISIS, LDP, MP-iBGP & Route Reflector

Complete Cisco IOS XR MPLS L3VPN Configuration Part 1

Complete Cisco IOS XR MPLS L3VPN Configuration Guide Part 1

In this complete MPLS L3VPN educational guide, we will configure a full Cisco IOS XR MPLS core environment using OSPF, ISIS, MPLS LDP, MP-iBGP, VPNv4, and Route Reflectors.

This article is designed for beginners, intermediate engineers, CCNP SP students, CCIE SP candidates, and enterprise engineers who want deep practical understanding of MPLS VPN architectures.

🎯 What You Will Learn

  • How MPLS works internally
  • OSPF configuration in IOS XR
  • ISIS configuration in IOS XR
  • MPLS LDP configuration
  • MP-iBGP VPNv4 architecture
  • Route Reflector concepts
  • Loopback design best practices
  • MPLS label distribution logic
  • VPNv4 route advertisement
  • Verification and troubleshooting

Table of Contents

1. MPLS Introduction

MPLS stands for Multiprotocol Label Switching.

Traditional IP routing performs hop-by-hop routing lookups using destination IP addresses.

MPLS improves forwarding efficiency using labels instead of repeated routing lookups.

MPLS Forwarding Formula

$$ Forwarding = Label\\ Lookup + LFIB $$

Where:

  • LFIB = Label Forwarding Information Base
  • Labels identify forwarding paths
  • MPLS reduces routing lookup complexity

IP Routing Complexity

$$ Routing\\ Complexity = O(n) $$

MPLS reduces repeated routing decisions by using label switching.

2. OSPF Configuration for AS100

OSPF is configured as the Interior Gateway Protocol for AS100.

OSPF distributes loopback and transit network reachability between routers.

Loopback interfaces are extremely important because:

  • They provide stable router IDs
  • They serve as BGP update-source interfaces
  • They improve network resiliency
  • They remain active even if physical interfaces fail

OSPF SPF Calculation

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

OSPF uses Dijkstra's SPF algorithm.

3. XR1 Configuration

XR1 belongs to AS100 and runs OSPF.

Code Example

router ospf 1

This initializes OSPF process 1. 

R1

hostname XR1

interface Gig0/0/0/0
 ip address 192.1.13.1 255.255.255.0
 no shut

interface Gig0/0/0/1
 ip address 192.1.100.1 255.255.255.0
 no shut

interface loopback0
 ip address 1.1.1.1 255.255.255.255

router ospf 1
 router-id 0.0.0.1
 area 0
  interface Gig0/0/0/0
  exit
  interface Gig0/0/0/1
  exit
  interface Loopback0
  exit

commit
Detailed Explanation

The command: 

router-id 0.0.0.1

defines a unique OSPF identifier.

Area 0 is the OSPF backbone area.

All interfaces participating in OSPF are added under Area 0.

Expected OSPF Neighbor Output 
XR1#show ospf neighbor

Neighbor ID     Pri State           Dead Time Address
0.0.0.3           1 FULL/DR         00:00:38 192.1.13.3

4. XR2 Configuration

R2

hostname XR2

interface Gig0/0/0/0
 ip address 192.1.23.2 255.255.255.0
 no shut

interface Gig0/0/0/1
 ip address 192.1.100.2 255.255.255.0
 no shut

interface loopback0
 ip address 2.2.2.2 255.255.255.255

router ospf 1
 router-id 0.0.0.2
 area 0
  interface Gig0/0/0/0
  exit
  interface Gig0/0/0/1
  exit
  interface Loopback0
  exit

commit
Why Use Loopbacks?

Loopbacks are always preferred for:

  • BGP peering
  • Router IDs
  • LDP Router IDs
  • MPLS stability

5. XR3 Configuration

R3

hostname XR3

interface Gig0/0/0/0
 ip address 192.1.13.3 255.255.255.0
 no shut

interface Gig0/0/0/1
 ip address 192.1.23.3 255.255.255.0
 no shut

interface loopback0
 ip address 3.3.3.3 255.255.255.255

router ospf 1
 router-id 0.0.0.3
 area 0
  interface Gig0/0/0/0
  exit
  interface Gig0/0/0/1
  exit
  interface Loopback0
  exit

commit

💡 Important Note

XR3 later becomes the Route Reflector for VPNv4 routes.

This reduces full-mesh iBGP requirements.

6. XR4 Configuration

R4

hostname XR4

interface Gig0/0/0/0
 ip address 192.1.34.4 255.255.255.0
 no shut

interface Gig0/0/0/1
 ip address 192.1.100.4 255.255.255.0
 no shut

interface Gig0/0/0/2
 ip address 192.1.34.3 255.255.255.0
 no shut

interface loopback0
 ip address 4.4.4.4 255.255.255.255

router ospf 1
 router-id 0.0.0.4
 area 0
  interface Gig0/0/0/0
  exit
  interface Gig0/0/0/1
  exit
  interface Loopback0
  exit

commit

7. MP-iBGP VPNv4 Configuration for AS100

MP-BGP distributes VPNv4 routes inside MPLS networks.

Standard IPv4 BGP cannot carry VPN labels and Route Targets.

VPNv4 extends BGP functionality.

VPNv4 Route Formula

$$ VPNv4 = RD + IPv4\\ Prefix $$

Example:

$$ 100:1 + 10.1.1.0/24 $$

R1 MP-iBGP Configuration

R1

router bgp 100
 address-family vpnv4 unicast
 exit

neighbor 3.3.3.3
 remote-as 100
 update-source Loopback0

 address-family vpnv4 unicast

R2 MP-iBGP Configuration

R2

router bgp 100
 address-family vpnv4 unicast
 exit

neighbor 3.3.3.3
 remote-as 100
 update-source Loopback0

 address-family vpnv4 unicast

R3 Route Reflector Configuration

R3

router bgp 100

 address-family vpnv4 unicast
 exit

neighbor-group MP-iBGP
 remote-as 100
 update-source Loopback0

 address-family vpnv4 unicast
  route-reflector-client
 exit
 exit

neighbor 1.1.1.1
 use neighbor-group MP-iBGP
 exit

neighbor 2.2.2.2
 use neighbor-group MP-iBGP
 exit

commit
What is a Route Reflector?

Normally iBGP requires full mesh connectivity.

The number of iBGP sessions grows rapidly:

$$ Sessions = \frac{n(n-1)}{2} $$

Where:

  • \(n\) = Number of routers

Route Reflectors eliminate this scaling issue.

iBGP Full Mesh Formula

$$ Sessions = \frac{n(n-1)}{2} $$

If:

$$ n=100 $$

Then:

$$ Sessions = 4950 $$

Route Reflectors dramatically reduce this number.

8. ISIS Configuration for AS200

ISIS is another Interior Gateway Protocol commonly used in Service Provider environments.

ISIS is preferred because:

  • It scales extremely well
  • It handles MPLS efficiently
  • It supports large provider cores
  • It converges quickly

ISIS NET Structure

$$ NET = Area\\ ID + System\\ ID + NSEL $$

9. XR5 Configuration

R5

hostname XR5

interface Gig0/0/0/0
 ip address 192.1.57.5 255.255.255.0
 no shut

interface Gig0/0/0/1
 ip address 192.1.200.5 255.255.255.0
 no shut

interface loopback0
 ip address 5.5.5.5 255.255.255.255

router isis 1
 net 49.0000.5555.5555.5555.00
 is-type level-2-only

 address-family ipv4 unicast
  metric-style wide
 exit

interface Gig0/0/0/0
 address-family ipv4 unicast
 exit
 exit

interface Gig0/0/0/1
 address-family ipv4 unicast
 exit
 exit

interface Loopback0
 address-family ipv4 unicast
 exit
 exit

commit
Why Wide Metrics?

Wide metrics extend ISIS scalability.

Traditional narrow metrics have limited values.

Wide metrics support Traffic Engineering and large topologies.

10. XR6 Configuration

R6

hostname XR6

interface Gig0/0/0/0
 ip address 192.1.67.6 255.255.255.0
 no shut

interface Gig0/0/0/1
 ip address 192.1.200.6 255.255.255.0
 no shut

interface loopback0
 ip address 6.6.6.6 255.255.255.255

router isis 1
 net 49.0000.6666.6666.6666.00
 is-type level-2-only

 address-family ipv4 unicast
  metric-style wide
 exit

interface Gig0/0/0/0
 address-family ipv4 unicast
 exit
 exit

interface Gig0/0/0/1
 address-family ipv4 unicast
 exit
 exit

interface Loopback0
 address-family ipv4 unicast
 exit
 exit

commit

11. XR7 Configuration

R7

hostname XR7

interface Gig0/0/0/0
 ip address 192.1.57.7 255.255.255.0
 no shut

interface Gig0/0/0/1
 ip address 192.1.67.7 255.255.255.0
 no shut

interface Gig0/0/0/2
 ip address 192.1.78.7 255.255.255.0
 no shut

interface loopback0
 ip address 7.7.7.7 255.255.255.255

router isis 1
 net 49.0000.7777.7777.7777.00
 is-type level-2-only

 address-family ipv4 unicast
  metric-style wide
 exit

interface Gig0/0/0/0
 address-family ipv4 unicast
 exit
 exit

interface Gig0/0/0/1
 address-family ipv4 unicast
 exit
 exit

interface Gig0/0/0/2
 address-family ipv4 unicast
 exit
 exit

interface Loopback0
 address-family ipv4 unicast
 exit
 exit

commit

12. XR8 Configuration

R8

hostname XR8

interface Gig0/0/0/0
 ip address 192.1.78.8 255.255.255.0
 no shut

interface Gig0/0/0/1
 ip address 192.1.200.8 255.255.255.0
 no shut

interface loopback0
 ip address 8.8.8.8 255.255.255.255

router isis 1
 net 49.0000.8888.8888.8888.00
 is-type level-2-only

 address-family ipv4 unicast
  metric-style wide
 exit

interface Gig0/0/0/0
 address-family ipv4 unicast
 exit
 exit

interface Gig0/0/0/1
 address-family ipv4 unicast
 exit
 exit

interface Loopback0
 address-family ipv4 unicast
 exit
 exit

commit

13. MPLS LDP Configuration

LDP distributes MPLS labels between routers.

Without LDP, MPLS forwarding cannot occur.

Label Switching Formula

$$ Incoming\\ Label \rightarrow Outgoing\\ Label $$

XR5 LDP Configuration

R5

mpls ldp
 router-id 5.5.5.5

 interface gig0/0/0/0
 exit

 interface gig0/0/0/1
 exit

commit

XR6 LDP Configuration

R6

mpls ldp
 router-id 6.6.6.6

 interface gig0/0/0/0
 exit

 interface gig0/0/0/1
 exit

commit

XR7 LDP Configuration

R7

mpls ldp
 router-id 7.7.7.7

 interface gig0/0/0/0
 exit

 interface gig0/0/0/1
 exit

 interface gig0/0/0/2
 exit

commit

XR8 LDP Configuration

R8

mpls ldp
 router-id 8.8.8.8

 interface gig0/0/0/0
 exit

 interface gig0/0/0/1
 exit

commit

14. MP-iBGP VPNv4 Configuration for AS200

XR5 Configuration

R5

router bgp 200

 address-family vpnv4 unicast
 exit

neighbor 7.7.7.7
 remote-as 200
 update-source Loopback0

 address-family vpnv4 unicast

commit

XR6 Configuration

R6

router bgp 200

 address-family vpnv4 unicast
 exit

neighbor 7.7.7.7
 remote-as 200
 update-source Loopback0

 address-family vpnv4 unicast

commit

XR7 Route Reflector Configuration

R7

router bgp 200

 address-family vpnv4 unicast
 exit

neighbor-group MP-iBGP
 remote-as 200
 update-source Loopback0

 address-family vpnv4 unicast
  route-reflector-client
 exit
 exit

neighbor 5.5.5.5
 use neighbor-group MP-iBGP
 exit

neighbor 6.6.6.6
 use neighbor-group MP-iBGP
 exit

commit

15. Verification Commands

Verify OSPF Neighbors

show ospf neighbor

Verify ISIS Neighbors

show isis neighbors

Verify MPLS LDP Neighbors

show mpls ldp neighbor

Verify VPNv4 BGP Sessions

show bgp vpnv4 unicast neighbors
Expected BGP Neighbor Output 
XR7#show bgp vpnv4 unicast neighbors

BGP neighbor is 5.5.5.5
 Remote AS 200
 BGP state = Established

16. Troubleshooting MPLS Networks

Problem Cause Solution
OSPF neighbor down Area mismatch Verify Area 0 configuration
ISIS adjacency missing NET issue Check NET addressing
LDP session failure IGP reachability missing Verify loopback reachability
BGP VPNv4 down Update-source missing Use Loopback0
No VPN routes VPNv4 AF missing Enable vpnv4 address-family

17. MPLS Mathematics and Networking Logic

End-to-End Label Switching

$$ Ingress\\ PE \rightarrow P\\ Router \rightarrow Egress\\ PE $$

Traffic Flow Equation

$$ Bandwidth = \frac{Data}{Time} $$

OSPF Cost Formula

$$ Cost = \frac{Reference\\ Bandwidth}{Interface\\ Bandwidth} $$

Label Distribution Logic

$$ FEC \rightarrow Label\\ Mapping $$

FEC means Forwarding Equivalence Class.

💡 Key Takeaways

  • OSPF and ISIS provide MPLS reachability
  • LDP distributes labels
  • Loopbacks are critical for stability
  • MP-BGP distributes VPNv4 routes
  • Route Reflectors reduce iBGP scaling problems
  • MPLS improves forwarding efficiency

Final Conclusion

This Part 1 MPLS L3VPN guide covered the complete MPLS underlay setup using Cisco IOS XR.

We configured:

  • OSPF Core
  • ISIS Core
  • MPLS LDP
  • MP-iBGP VPNv4
  • Route Reflectors
  • Loopback reachability

These technologies form the foundation of large-scale MPLS service provider networks.

In upcoming parts, VRFs, Route Targets, VPN customers, and MPLS Layer 3 VPN services can be added on top of this infrastructure.

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