Complete MPLS Inter-AS VPN Lab Part 2 | OSPF, IS-IS, MPLS LDP & MP-BGP Configuration Guide

Complete MPLS Inter-AS VPN Lab Part 2

This is Part 2 of the MPLS Inter-AS VPN Lab Series. In this tutorial we will build the underlay routing infrastructure using OSPF and IS-IS, configure a Backbone Service Provider using MPLS VPN, establish MP-BGP VPNv4 communication and verify end-to-end connectivity between the AS 100 US and UK networks.

📘 This Article Is Part 2

Before continuing, read Part 1:

• Complete MPLS Inter-AS VPN Options B & C Configuration Guide (Part 1)

Table of Contents

• 1. MPLS Inter-AS Architecture Overview
• 2. OSPF Underlay Configuration
• 3. IS-IS Underlay Configuration
• 4. MPLS Backbone Provider Configuration
• 5. MP-BGP VPNv4 Configuration
• 6. MPLS LDP Explained
• 7. Verification Commands
• 8. MPLS and Routing Mathematics
• 9. Troubleshooting
• 10. Best Practices
• 11. Related Articles

1. MPLS Inter-AS Architecture Overview

The topology consists of:

• AS 100 US Network using OSPF
• AS 100 UK Network using IS-IS
• AS 1000 acting as Backbone Service Provider
• MPLS VPN connectivity between both sites

      AS 100 (US)                  AS 1000 Backbone               AS 100 (UK)

  R1 ----- R3 ----- R4 ====== R13 ====== R14 ====== R8 ----- R7 ----- R5
    \\         \\                                                /
      \\         R2 -------------------------------------------- R6

MPLS VPN End-to-End Formula

$$ Connectivity = Underlay + MPLS + VPNv4 + RT\\ Matching $$

All components must function correctly for VPN communication.

2. Configure OSPF Underlay for AS 100 (US)

OSPF provides underlay routing between routers R1, R2, R3 and R4.

All routers use:

• Area 0
• Router-ID x.x.x.x
• Internal link advertisements

OSPF SPF Formula

$$ Shortest\\ Path = \sum Cost $$

OSPF calculates the shortest path using Dijkstra's SPF algorithm.

Task 1 - Configure OSPF on R1

Code Example

router ospf 1

R1

router ospf 1
 router-id 1.1.1.1
 network 1.1.1.1 0.0.0.0 area 0
 network 192.1.13.0 0.0.0.255 area 0
 network 192.1.100.0 0.0.0.255 area 0

Configure OSPF on R2

R2

router ospf 1
 router-id 2.2.2.2
 network 2.2.2.2 0.0.0.0 area 0
 network 192.1.23.0 0.0.0.255 area 0
 network 192.1.100.0 0.0.0.255 area 0

Configure OSPF on R3

R3

router ospf 1
 router-id 3.3.3.3
 network 3.3.3.3 0.0.0.0 area 0
 network 192.1.13.0 0.0.0.255 area 0
 network 192.1.24.0 0.0.0.255 area 0
 network 192.1.34.0 0.0.0.255 area 0

Configure OSPF on R4

R4

router ospf 1
 router-id 4.4.4.4
 network 4.4.4.4 0.0.0.0 area 0
 network 192.1.34.0 0.0.0.255 area 0
 network 192.1.100.0 0.0.0.255 area 0

Detailed OSPF Explanation

The router-id uniquely identifies each OSPF router.

The network statements:

• Enable OSPF on interfaces
• Advertise connected networks
• Form adjacencies with neighbors

All routers belong to Area 0 which is the OSPF backbone area.

OSPF Cost Calculation

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

Default Cisco reference bandwidth:

$$ 100Mbps $$

3. Configure IS-IS Underlay for AS 100 (UK)

IS-IS is configured between routers R5, R6, R7 and R8.

We use:

• Level-2 routing
• Wide metric style
• Area ID 49.0000

IS-IS NET Formula

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

Example:

$$ 49.0000.5555.5555.5555.00 $$

Configure IS-IS on R5

R5

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

interface E0/0
 ip router isis

Configure IS-IS on R6

R6

router isis
 net 49.0000.6666.6666.6666.00
 is-type level-2
 metric-style wide
!

interface E0/0
 ip router isis
!

interface E0/1
 ip router isis
!

interface Loopback0
 ip router isis

Configure IS-IS on R7

R7

router isis
 net 49.0000.7777.7777.7777.00
 is-type level-2
 metric-style wide
!

interface E0/0
 ip router isis
!

interface E0/1
 ip router isis
!

interface E0/2
 ip router isis
!

interface Loopback0
 ip router isis

Configure IS-IS on R8

R8

router isis
 net 49.0000.8888.8888.8888.00
 is-type level-2
 metric-style wide
!

interface E0/0
 ip router isis
!

interface E0/1
 ip router isis
!

interface Loopback0
 ip router isis

Why Use Wide Metrics?

Wide metrics support larger metric values and modern MPLS deployments.

Traditional narrow metrics are limited and less scalable.

IS-IS SPF Formula

$$ Best\\ Path = Lowest\\ Metric\\ Sum $$

4. Configure MPLS Backbone Service Provider

AS 1000 acts as a Backbone Service Provider connecting both AS 100 sites.

The provider uses:

• EIGRP underlay routing
• MPLS LDP
• VPNv4 MP-BGP
• VRFs for customer isolation

Configure Backbone Router R13

R13

router eigrp 100
 network 13.0.0.0
 network 192.1.134.0
!

mpls ldp router-id Loopback0
!

interface E0/1
 mpls ip
!

router bgp 1000
 neighbor 14.14.14.14 remote-as 1000
 neighbor 14.14.14.14 update-source Loopback0
!

address-family vpnv4
 neighbor 14.14.14.14 activate
!

vrf definition SP-100
 rd 1000:1

 address-family ipv4
  route-target both 1000:1
!

interface E0/0
 vrf forwarding SP-100
 ip address 192.1.40.13 255.255.255.0
 no shut
!

router bgp 1000
 address-family ipv4 vrf SP-100
  neighbor 192.1.40.4 remote-as 100

Configure Backbone Router R14

R14

router eigrp 100
 network 13.0.0.0
 network 192.1.134.0
!

mpls ldp router-id Loopback0
!

interface E0/1
 mpls ip
!

router bgp 1000
 neighbor 13.13.13.13 remote-as 1000
 neighbor 13.13.13.13 update-source Loopback0
!

address-family vpnv4
 neighbor 13.13.13.13 activate
!

vrf definition SP-100
 rd 1000:1

 address-family ipv4
  route-target both 1000:1
!

interface E0/0
 vrf forwarding SP-100
 ip address 192.1.80.14 255.255.255.0
 no shut
!

router bgp 1000
 address-family ipv4 vrf SP-100
  neighbor 192.1.80.8 remote-as 100

MPLS Label Switching Formula

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

Routers forward packets using labels instead of long IP lookups.

5. Configure PE-CE BGP Connectivity

Configure R4

R4

interface E0/2
 ip address 192.1.40.4 255.255.255.0
 no shut
!

router bgp 100
 neighbor 192.1.40.13 remote-as 1000
 neighbor 192.1.40.13 allowas-in
 redistribute ospf 1
!

router ospf 1
 redistribute bgp 100

Configure R8

R8

interface E0/2
 ip address 192.1.80.8 255.255.255.0
 no shut
!

router bgp 100
 neighbor 192.1.80.14 remote-as 1000
 neighbor 192.1.80.14 allowas-in
 redistribute isis
!

router isis
 redistribute bgp 100

Understanding allowas-in

Normally BGP rejects routes containing its own AS number.

The command:

allowas-in

permits routes with the local AS in the AS-path.

This is common in MPLS Inter-AS environments.

BGP AS Path Formula

$$ Best\\ Path = Shortest\\ AS\\ Path $$

BGP prefers routes with fewer AS hops.

6. MPLS LDP Explained

MPLS LDP distributes labels between MPLS routers.

The command:

mpls ip

enables MPLS forwarding on interfaces.

The command:

mpls ldp router-id loopback0

defines a stable LDP identifier.

LDP Label Mapping Formula

$$ FEC \rightarrow MPLS\\ Label $$

FEC stands for Forwarding Equivalence Class.

7. Verification Commands

Command	Purpose
show ip ospf neighbor	Verify OSPF adjacency
show isis neighbors	Verify IS-IS adjacency
show mpls forwarding-table	Verify MPLS labels
show bgp vpnv4 unicast all	Verify VPNv4 routes
show ip route	Verify routing table
ping x.x.x.x	Verify connectivity
traceroute x.x.x.x	Verify packet path

Sample MPLS Verification Output

R13#show mpls forwarding-table

Local  Outgoing Prefix
16     Pop Tag  14.14.14.14/32
17     22       192.1.80.0/24

Expected Ping Verification

R1#ping 8.8.8.8 source lo0

!!!!!
Success rate is 100 percent

8. MPLS and Routing Mathematics

VPNv4 Formula

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

OSPF SPF Tree Formula

$$ Shortest\\ Path = Minimum\\ Total\\ Cost $$

MPLS Encapsulation Formula

$$ Packet = IP + MPLS\\ Label $$

BGP Route Selection Formula

$$ Best\\ Path = Weight + LocalPref + ASPath $$

Label Stack Formula

$$ Top\\ Label + VPN\\ Label + Payload $$

9. Troubleshooting MPLS VPN Connectivity

Problem	Cause	Solution
No OSPF neighbors	Area mismatch	Verify Area 0
No IS-IS adjacency	Missing ip router isis	Enable IS-IS on interfaces
No MPLS labels	MPLS disabled	Enable mpls ip
No VPNv4 routes	BGP AF missing	Activate vpnv4 AF
BGP routes rejected	AS-path loop	Use allowas-in

10. Best Practices

• Use Loopback interfaces for stable router IDs
• Always enable MPLS only on backbone links
• Use route filtering carefully
• Document Route Targets clearly
• Use consistent IGP metrics
• Verify MPLS labels frequently
• Use wide metrics in IS-IS deployments
• Keep MPLS backbone isolated from customer traffic

💡 Key Takeaways

• OSPF provides the US underlay routing
• IS-IS provides the UK underlay routing
• AS 1000 acts as MPLS VPN Backbone SP
• MPLS LDP distributes transport labels
• VPNv4 MP-BGP exchanges customer routes
• allowas-in permits local AS path acceptance
• Traceroute confirms packets remain IP within customer AS

11. Related Articles

• Complete MPLS Inter-AS VPN Options B & C Configuration Guide (Part 1)
• Complete MPLS Inter-AS VPN Lab Part 3 | MPLS LDP & BGP Send-Label Configuration Guide

Final Conclusion

This MPLS Inter-AS VPN Lab Part 2 tutorial demonstrated how to build a complete MPLS VPN backbone connecting multiple autonomous systems using OSPF, IS-IS, MPLS LDP and MP-BGP VPNv4.

We configured:

• OSPF underlay routing
• IS-IS underlay routing
• MPLS VPN Backbone SP
• VRFs and Route Targets
• VPNv4 MP-BGP
• PE-CE BGP relationships
• MPLS label switching
• End-to-end VPN connectivity

Understanding MPLS Inter-AS VPN architecture is critical for service provider and enterprise WAN deployments where scalability, segmentation and transport efficiency are required.