Thursday, May 14, 2026

Complete MPLS L3VPN Multi-IGP Configuration Guide | OSPF, IS-IS, EIGRP, MP-BGP & VRF Lab

Complete MPLS L3VPN Multi-IGP Lab Configuration Guide

Complete MPLS L3VPN Multi-IGP Configuration Guide

This complete MPLS L3VPN tutorial explains how to build an enterprise-grade MPLS backbone using multiple Interior Gateway Protocols including OSPF, IS-IS and EIGRP. The lab also demonstrates MPLS LDP configuration, Route Reflectors, MP-BGP, VPNv4 route exchange and PE-CE connectivity.

🎯 What You Will Learn

  • OSPF MPLS Core Configuration
  • IS-IS MPLS Backbone Setup
  • EIGRP MPLS Transport Configuration
  • LDP Label Distribution
  • BGP Route Reflector Design
  • MP-BGP VPNv4 Route Exchange
  • VRF Configuration
  • PE-CE Routing
  • Customer Route Advertisement
  • MPLS Label Switching Logic
  • VPN Route Propagation
  • Verification and Troubleshooting

Table of Contents

1. MPLS L3VPN Overview

MPLS stands for Multiprotocol Label Switching.

MPLS uses labels instead of traditional routing lookups.

This allows:

  • Faster forwarding
  • Traffic engineering
  • VPN services
  • QoS integration
  • Scalable provider networks

MPLS Forwarding Formula

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

Where:

  • LFIB = Label Forwarding Information Base
  • Labels identify forwarding paths

2. Network Topology


        OSPF DOMAIN
PE1 -------- P1 -------- RR1

                         |
                         |
                    IS-IS DOMAIN

RR1 -------- P2 -------- RR2

                         |
                         |
                    EIGRP DOMAIN

RR2 -------- P3 -------- PE2

The MPLS backbone contains:

  • OSPF Area 0 Domain
  • IS-IS Level-2 Domain
  • EIGRP AS 100 Domain
  • Route Reflectors
  • VPNv4 MP-BGP

3. OSPF MPLS Domain Configuration

The first MPLS segment uses OSPF as the IGP.

PE1, P1 and RR1 belong to this domain.

OSPF Cost Formula

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

PE1 Configuration

Code Example

mpls ldp router-id loopback0

hostname PE1

interface E0/1
 ip address 192.168.10.1 255.255.255.0
 no shutdown

interface loopback0
 ip address 10.1.1.1 255.255.255.255

router ospf 1
 network 10.1.1.0 0.0.0.255 area 0
 network 192.168.10.0 0.0.0.255 area 0

mpls ldp router-id loopback0

interface E0/1
 mpls ip
Detailed PE1 Explanation

The loopback interface becomes the LDP Router-ID.

OSPF advertises all core interfaces.

The command: 

mpls ip

enables MPLS forwarding on the interface.

P1 Configuration

hostname P1

interface E0/0
 ip address 192.168.10.2 255.255.255.0
 no shutdown

interface E0/1
 ip address 192.168.20.2 255.255.255.0
 no shutdown

interface loopback0
 ip address 10.1.1.2 255.255.255.255

router ospf 1
 network 10.1.1.0 0.0.0.255 area 0
 network 192.168.10.0 0.0.0.255 area 0
 network 192.168.20.0 0.0.0.255 area 0

mpls ldp router-id loopback0

interface E0/0
 mpls ip

interface E0/1
 mpls ip

RR1 Configuration

hostname RR1

interface E0/0
 ip address 192.168.20.3 255.255.255.0
 no shutdown

interface loopback0
 ip address 10.1.1.3 255.255.255.255

router ospf 1
 network 10.1.1.0 0.0.0.255 area 0
 network 192.168.20.0 0.0.0.255 area 0

mpls ldp router-id loopback0

interface E0/0
 mpls ip

4. IS-IS MPLS Domain Configuration

The second MPLS segment uses IS-IS.

IS-IS is commonly used by service providers because of scalability.

IS-IS SPF Formula

$$ Shortest\\ Path = \sum Link\\ Metrics $$

RR1 IS-IS Configuration

interface E0/1
 ip address 192.168.30.3 255.255.255.0
 no shutdown

router isis
 net 49.0000.3333.3333.3333.00
 is-type level-2
 metric-style wide

interface loopback0
 ip router isis

interface E0/1
 ip router isis
 mpls ip
Understanding IS-IS NET Address

NET stands for Network Entity Title.

Example: 

49.0000.3333.3333.3333.00
  • 49 = Private AFI
  • 0000 = Area ID
  • 3333.3333.3333 = System ID
  • 00 = NSEL

P2 Configuration

hostname P2

interface E0/0
 ip address 192.168.30.4 255.255.255.0
 no shutdown

interface E0/1
 ip address 192.168.40.4 255.255.255.0
 no shutdown

interface loopback0
 ip address 10.1.1.4 255.255.255.255

router isis
 net 49.0000.4444.4444.4444.00
 is-type level-2
 metric-style wide

mpls ldp router-id loopback0

interface E0/0
 ip router isis
 mpls ip

interface E0/1
 ip router isis
 mpls ip

interface loopback0
 ip router isis

RR2 Configuration

hostname RR2

interface E0/0
 ip address 192.168.40.5 255.255.255.0
 no shutdown

interface loopback0
 ip address 10.1.1.5 255.255.255.255

router isis
 net 49.0000.5555.5555.5555.00
 is-type level-2
 metric-style wide

mpls ldp router-id loopback0

interface E0/0
 ip router isis
 mpls ip

interface loopback0
 ip router isis

5. EIGRP MPLS Domain Configuration

The third MPLS domain uses EIGRP.

EIGRP Metric Formula

$$ Metric = 256 \times \left( \frac{10^7}{Bandwidth} + Delay \right) $$

RR2 EIGRP Configuration

interface E0/1
 ip address 192.168.50.5 255.255.255.0
 no shutdown

router eigrp 100
 network 192.168.50.0
 network 10.1.1.0 0.0.0.255

interface E0/1
 mpls ip

P3 Configuration

hostname P3

interface E0/0
 ip address 192.168.50.6 255.255.255.0
 no shutdown

interface E0/1
 ip address 192.168.60.6 255.255.255.0
 no shutdown

interface loopback0
 ip address 10.1.1.6 255.255.255.255

router eigrp 100
 network 192.168.50.0
 network 192.168.60.0
 network 10.1.1.0 0.0.0.255

mpls ldp router-id loopback0

interface E0/0
 mpls ip

interface E0/1
 mpls ip

PE2 Configuration

hostname PE2

interface E0/0
 ip address 192.168.60.7 255.255.255.0
 no shutdown

interface loopback0
 ip address 10.1.1.7 255.255.255.255

router eigrp 100
 network 192.168.60.0
 network 10.1.1.0 0.0.0.255

mpls ldp router-id loopback0

interface E0/0
 mpls ip

6. BGP Label Exchange Configuration

Now we configure BGP labeled unicast sessions.

The Route Reflectors reduce iBGP full mesh requirements.

BGP Scaling Formula

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

Route Reflectors dramatically reduce this complexity.

PE1 BGP Configuration

router bgp 100
 network 10.1.1.1 mask 255.255.255.255

 neighbor 10.1.1.3 remote-as 100
 neighbor 10.1.1.3 update-source loopback0
 neighbor 10.1.1.3 send-label

RR1 BGP Configuration

router bgp 100

 neighbor 10.1.1.1 remote-as 100
 neighbor 10.1.1.1 update-source loopback0
 neighbor 10.1.1.1 route-reflector-client
 neighbor 10.1.1.1 next-hop-self all
 neighbor 10.1.1.1 send-label

 neighbor 10.1.1.5 remote-as 100
 neighbor 10.1.1.5 update-source loopback0
 neighbor 10.1.1.5 route-reflector-client
 neighbor 10.1.1.5 next-hop-self all
 neighbor 10.1.1.5 send-label

RR2 BGP Configuration

router bgp 100

 neighbor 10.1.1.3 remote-as 100
 neighbor 10.1.1.3 update-source loopback0
 neighbor 10.1.1.3 route-reflector-client
 neighbor 10.1.1.3 next-hop-self all
 neighbor 10.1.1.3 send-label

 neighbor 10.1.1.7 remote-as 100
 neighbor 10.1.1.7 update-source loopback0
 neighbor 10.1.1.7 route-reflector-client
 neighbor 10.1.1.7 next-hop-self all
 neighbor 10.1.1.7 send-label

PE2 BGP Configuration

router bgp 100
 network 10.1.1.7 mask 255.255.255.255

 neighbor 10.1.1.5 remote-as 100
 neighbor 10.1.1.5 update-source loopback0
 neighbor 10.1.1.5 send-label
What Does send-label Do?

The command: 

neighbor X.X.X.X send-label

enables labeled IPv4 route advertisement.

This allows MPLS labels to be carried through BGP.

7. MP-BGP VPNv4 Configuration

Now we establish MP-iBGP VPNv4 sessions.

IPv4 unicast routes are blocked using prefix-lists.

VPNv4 Route Formula

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

PE1 VPNv4 Configuration

ip prefix-list DENYALL deny 0.0.0.0/0 le 32

router bgp 100

 neighbor 10.1.1.7 remote-as 100
 neighbor 10.1.1.7 update-source loopback0
 neighbor 10.1.1.7 prefix-list DENYALL out

 address-family vpnv4
  neighbor 10.1.1.7 activate

PE2 VPNv4 Configuration

ip prefix-list DENYALL deny 0.0.0.0/0 le 32

router bgp 100

 neighbor 10.1.1.1 remote-as 100
 neighbor 10.1.1.1 update-source loopback0
 neighbor 10.1.1.1 prefix-list DENYALL out

 address-family vpnv4
  neighbor 10.1.1.1 activate

8. VRF and PE-CE Routing

VRFs isolate customer routing tables.

Customer routes are exchanged through MP-BGP.

VRF Isolation Formula

$$ CustomerA\\ Routes \neq CustomerB\\ Routes $$

PE1 VRF Configuration

vrf definition CUST-A
 rd 100:1

 address-family ipv4
  route-target both 100:1

interface E0/0
 vrf forwarding CUST-A
 ip address 192.168.1.1 255.255.255.0
 no shutdown

router bgp 100

 address-family ipv4 vrf CUST-A
  neighbor 192.168.1.11 remote-as 65001

PE2 VRF Configuration

vrf definition CUST-A
 rd 100:1

 address-family ipv4
  route-target both 100:1

interface E0/1
 vrf forwarding CUST-A
 ip address 192.168.2.7 255.255.255.0
 no shutdown

router bgp 100

 address-family ipv4 vrf CUST-A
  neighbor 192.168.2.22 remote-as 65002

CE1 Configuration

interface E0/0
 ip address 192.168.1.11 255.255.255.0
 no shutdown

interface loopback0
 ip address 10.11.11.11 255.255.255.0

router bgp 65001
 neighbor 192.168.1.1 remote-as 100
 network 10.11.11.0 mask 255.255.255.0

CE2 Configuration

interface E0/0
 ip address 192.168.2.22 255.255.255.0
 no shutdown

interface loopback0
 ip address 10.22.22.22 255.255.255.0

router bgp 65002
 neighbor 192.168.2.7 remote-as 100
 network 10.22.22.0 mask 255.255.255.0

9. Verification Commands

Verify LDP Neighbors

show mpls ldp neighbor

Verify MPLS Interfaces

show mpls interfaces

Verify OSPF Neighbors

show ip ospf neighbor

Verify IS-IS Neighbors

show isis neighbors

Verify EIGRP Neighbors

show ip eigrp neighbors

Verify VPNv4 Routes

show bgp vpnv4 unicast all

Verify VRF Routing

show ip route vrf CUST-A
Expected Ping Verification 
CE1# ping 10.22.22.22

!!!!!
Success rate is 100 percent

10. MPLS Mathematical Concepts

Label Switching Formula

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

VPN Route Propagation Formula

$$ Import = Export\\ RT \cap Import\\ RT $$

BGP Best Path Simplified

$$ Best\\ Path = Highest\\ LocalPref + Shortest\\ AS\\ Path $$

Scalability Mathematics

$$ Total\\ Labels = Number\\ of\\ FECs $$

Where FEC means Forwarding Equivalence Class.

💡 Key Takeaways

  • MPLS separates forwarding from routing
  • LDP distributes transport labels
  • OSPF, IS-IS and EIGRP can all transport MPLS
  • MP-BGP distributes VPN routes
  • Route Reflectors improve scalability
  • VRFs isolate customer traffic
  • VPNv4 enables overlapping address spaces

Final Conclusion

This MPLS L3VPN multi-IGP deployment demonstrated how service provider networks can integrate OSPF, IS-IS and EIGRP together while transporting MPLS labels end-to-end.

The lab also covered:

  • LDP Label Distribution
  • BGP labeled unicast
  • Route Reflectors
  • MP-BGP VPNv4
  • VRF configuration
  • PE-CE routing
  • Customer route propagation

This type of architecture is commonly used in large-scale enterprise and ISP networks.

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