Thursday, May 14, 2026

Complete MPLS Inter-AS VPN Lab Part 3 | MPLS LDP & BGP Send-Label Configuration Guide

Complete MPLS Inter-AS VPN Lab Part 3 | MPLS LDP and BGP Labeled Unicast

Complete MPLS Inter-AS VPN Lab Part 3

Welcome to Part 3 of the Complete MPLS Inter-AS VPN Lab Series.

In this section we focus on one of the most important technologies in MPLS environments:

MPLS LDP
Label Distribution
BGP Labeled Unicast
send-label capability
End-to-End MPLS forwarding

1. MPLS LDP Introduction
2. Configure MPLS LDP in AS 100 US
3. Configure MPLS LDP in AS 100 UK
4. Configure BGP send-label
5. Verification Commands
6. MPLS Label Mathematics
7. Troubleshooting
8. Best Practices
9. Related Articles

1. MPLS LDP Introduction

MPLS stands for Multi-Protocol Label Switching.

Instead of forwarding packets purely based on IP routing lookups, MPLS uses labels to make forwarding decisions.

LDP (Label Distribution Protocol) distributes labels between routers.

MPLS Core Formula

$$ Packet\\ Forwarding = Label\\ Lookup $$

In traditional IP routing:

$$ Forwarding = Destination\\ IP\\ Lookup $$

In MPLS:

$$ Forwarding = MPLS\\ Label $$

This significantly improves forwarding efficiency and scalability.

💡 Key MPLS Components

LER = Label Edge Router
LSR = Label Switch Router
LDP = Label Distribution Protocol
FEC = Forwarding Equivalence Class
LFIB = Label Forwarding Information Base

2. Configure MPLS LDP within AS 100 US Site

Routers involved:

The command:

mpls ldp router-id loopback0

defines a stable LDP identifier.

The command:

mpls ip

enables MPLS forwarding on interfaces.

Label Distribution Formula

$$ FEC \rightarrow Label $$

Configure MPLS on R1

Code Example

mpls ldp router-id loopback0

R1

mpls ldp router-id loopback0
!

interface E0/0
 mpls ip
!

interface E0/1
 mpls ip

Configure MPLS on R2

R2

mpls ldp router-id loopback0
!

interface E0/0
 mpls ip
!

interface E0/1
 mpls ip

Configure MPLS on R3

R3

mpls ldp router-id loopback0
!

interface E0/0
 mpls ip
!

interface E0/1
 mpls ip
!

interface E0/2
 mpls ip

Configure MPLS on R4

R4

mpls ldp router-id loopback0
!

interface E0/0
 mpls ip
!

interface E0/1
 mpls ip

How LDP Neighbors Form

LDP neighbors form automatically over MPLS-enabled interfaces.

Requirements:

IP connectivity
IGP reachability
MPLS enabled on interfaces

LDP uses TCP port 646 for session establishment.

LDP Session Formula

$$ IGP\\ Reachability + MPLS\\ Enabled = LDP\\ Neighbor $$

3. Configure MPLS LDP within AS 100 UK Site

Now we configure MPLS LDP in the UK Site.

Routers:

Configure MPLS on R5

R5

mpls ldp router-id loopback0
!

interface E0/0
 mpls ip
!

interface E0/1
 mpls ip

Configure MPLS on R6

R6

mpls ldp router-id loopback0
!

interface E0/0
 mpls ip
!

interface E0/1
 mpls ip

Configure MPLS on R7

R7

mpls ldp router-id loopback0
!

interface E0/0
 mpls ip
!

interface E0/1
 mpls ip
!

interface E0/2
 mpls ip

Configure MPLS on R8

R8

mpls ldp router-id loopback0
!

interface E0/0
 mpls ip
!

interface E0/1
 mpls ip

MPLS Label Switching Formula

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

What Happens During MPLS Forwarding?

When a packet enters the MPLS cloud:

Ingress router pushes a label
Transit routers swap labels
Egress router removes labels

This process is called:

$$ Push \rightarrow Swap \rightarrow Pop $$

4. Configure PE-CE Routing with BGP send-label

The send-label command allows BGP to exchange MPLS labels together with routes.

This is required for:

BGP labeled unicast
Inter-AS MPLS VPNs
End-to-End MPLS forwarding

BGP Labeled Route Formula

$$ BGP\\ Route + MPLS\\ Label $$

Configure send-label on R4

Code Example

neighbor 192.1.40.13 send-label

R4

router bgp 100
 neighbor 192.1.40.13 send-label

Configure send-label on R8

R8

router bgp 100
 neighbor 192.1.80.14 send-label

Configure send-label on R13

R13

router bgp 1000

 address-family ipv4 vrf SP-100
  neighbor 192.1.40.4 send-label

Configure send-label on R14

R14

router bgp 1000

 address-family ipv4 vrf SP-100
  neighbor 192.1.80.8 send-label

Why send-label Is Important

Without send-label:

BGP exchanges only prefixes
No MPLS labels are exchanged
Packets become plain IP packets

With send-label:

BGP exchanges routes and labels
MPLS forwarding continues end-to-end
Traffic remains labeled across AS boundaries

End-to-End MPLS Formula

$$ LDP + BGP\\ send-label = End\\ to\\ End\\ MPLS $$

5. Verification Commands

Command	Purpose
show mpls ldp neighbor	Verify LDP neighbors
show mpls forwarding-table	Verify MPLS labels
show bgp ipv4 labeled-unicast	Verify labeled BGP routes
show ip cef	Verify forwarding entries
ping	Verify connectivity
traceroute	Verify MPLS labels

Sample LDP Neighbor Output

R1#show mpls ldp neighbor

Peer LDP Identifier: 3.3.3.3:0
TCP connection: 3.3.3.3.646
State: Oper

Sample MPLS Forwarding Table

R3#show mpls forwarding-table

Local  Outgoing  Prefix
16     18        8.8.8.8/32
17     Pop Tag   4.4.4.4/32

Sample Traceroute Showing Labels

R1#traceroute 8.8.8.8

1 192.1.13.3 [MPLS: Label 18 Exp 0]
2 192.1.34.4 [MPLS: Label 22 Exp 0]
3 192.1.40.13 [MPLS: Label 30 Exp 0]

🎯 Verification Goals

Ping between R1 and R8 loopbacks must succeed
MPLS labels should appear in traceroute
LDP neighbors must be operational
BGP labeled routes must be exchanged

6. MPLS Label Mathematics

MPLS Label Stack Formula

$$ Transport\\ Label + VPN\\ Label + Payload $$

Push Operation

$$ IP\\ Packet \rightarrow Add\\ Label $$

Swap Operation

$$ Old\\ Label \rightarrow New\\ Label $$

Pop Operation

$$ Remove\\ MPLS\\ Label $$

Forwarding Equation

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

7. Troubleshooting MPLS LDP

Problem	Cause	Solution
No LDP neighbors	MPLS disabled	Enable mpls ip
No labels	No IGP reachability	Verify routing
No labeled traceroute	Missing send-label	Configure send-label
Ping failure	No label path	Verify forwarding table
BGP labels missing	AF mismatch	Verify address-family

8. Best Practices

Use Loopbacks as LDP router IDs
Enable MPLS only on core links
Verify LDP neighbors before BGP
Always verify label forwarding tables
Use traceroute to verify MPLS operations
Keep IGP stable before enabling MPLS
Use send-label carefully in Inter-AS deployments

💡 Key Takeaways

LDP distributes MPLS labels
MPLS forwarding uses labels instead of IP lookups
BGP send-label enables labeled route exchange
End-to-end MPLS requires labels across all AS boundaries
Traceroute can confirm MPLS label switching

Final Conclusion

In Part 3 we configured MPLS LDP across both AS 100 sites and enabled BGP labeled unicast using the send-label capability.

We successfully achieved:

MPLS label distribution
LDP neighbor establishment
BGP labeled route exchange
End-to-end MPLS forwarding
Labeled packet traceroute verification

This completes a critical stage in building scalable MPLS Inter-AS VPN architectures used by real-world service providers.

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Thursday, May 14, 2026