Berik Ashimov
Senior IT Engineer

BGP-LU: Labeled Unicast for Scalable MPLS Networks

· 6 min read
#vyos #bgp #mpls #networking

LDP distributes labels within an autonomous system. But LDP doesn’t cross AS boundaries. For MPLS across multiple ASes, you need something else.

BGP Labeled Unicast (BGP-LU) distributes MPLS labels via BGP — the same protocol already handling inter-AS routing. Labels follow prefixes across AS boundaries, enabling end-to-end MPLS paths spanning multiple networks.

BGP-LU replaces LDP in modern scalable designs.

Why BGP-LU

LDP Limitations

AS 65001              AS 65002
[PE1]──[P1]──[ASBR1]  [ASBR2]──[P2]──[PE2]
     LDP               │         LDP
                       │
                   eBGP session
                   (no labels!)

LDP sessions don’t cross AS boundaries. MPLS stops at the border.

BGP-LU Solution

AS 65001              AS 65002
[PE1]──[P1]──[ASBR1]══[ASBR2]──[P2]──[PE2]
                   BGP-LU
              (prefixes + labels)

BGP carries labels along with prefixes. MPLS continues across ASes.

BGP-LU Basics

How It Works

1. PE1 allocates label for its loopback (10.255.0.1/32)
2. PE1 advertises via BGP: 10.255.0.1/32, label 1001
3. Intermediate routers receive prefix+label
4. Traffic to 10.255.0.1 gets labeled 1001 at ingress
5. Label-switched to PE1

BGP-LU vs LDP

FeatureLDPBGP-LU
ScopeSingle ASMulti-AS
ProtocolSeparate (LDP)Existing (BGP)
Label bindingFEC-basedPrefix-based
ScalabilityIGP-limitedBGP-limited
OperationalTwo protocolsOne protocol

Configuring BGP-LU

Enable Labeled Unicast Address Family

Terminal window
configure


# Enable labeled unicast for IPv4
set protocols bgp address-family ipv4-labeled-unicast


# Redistribute connected/loopback with labels
set protocols bgp address-family ipv4-labeled-unicast redistribute connected


commit

Configure BGP-LU Neighbor

Terminal window
configure


# iBGP neighbor with labeled unicast
set protocols bgp neighbor 10.255.0.2 remote-as 65001
set protocols bgp neighbor 10.255.0.2 address-family ipv4-labeled-unicast


# eBGP neighbor with labeled unicast
set protocols bgp neighbor 192.0.2.1 remote-as 65002
set protocols bgp neighbor 192.0.2.1 address-family ipv4-labeled-unicast


commit

Network Statement with Labels

Terminal window
configure


# Advertise loopback with label
set protocols bgp address-family ipv4-labeled-unicast network 10.255.0.1/32


commit

Viewing BGP-LU State

Show Labeled Routes

Terminal window
# Show BGP-LU routes
show bgp ipv4 labeled-unicast


# Output:
# Network          Next Hop        Label      Path
# 10.255.0.1/32    0.0.0.0         1001       i
# 10.255.0.2/32    10.0.0.2        2001       i
# 10.255.0.3/32    192.0.2.1       3001       65002 i

Show Label Bindings

Terminal window
# Show MPLS table (FRR)
show mpls table


# Show specific prefix label
show bgp ipv4 labeled-unicast 10.255.0.2/32

Inter-AS MPLS Options

Option A: Back-to-Back VRF

No BGP-LU needed:
[PE1]──MPLS──[ASBR1]─────VRF─────[ASBR2]──MPLS──[PE2]
                    IP routing
                    (MPLS restarts)

Simple but doesn’t provide end-to-end MPLS.

Option B: eBGP Labeled Unicast

BGP-LU across ASBRs:
[PE1]──MPLS──[ASBR1]═══BGP-LU═══[ASBR2]──MPLS──[PE2]
Terminal window
# ASBR1 config
set protocols bgp neighbor 192.0.2.1 remote-as 65002
set protocols bgp neighbor 192.0.2.1 address-family ipv4-labeled-unicast


# ASBR1 must swap labels at boundary

Option C: Multihop eBGP + Labels

BGP-LU between PEs (via route reflector):
[PE1]════════════BGP-LU═════════════[PE2]
        (reflected through ASBRs)

Most scalable, but complex.

Seamless MPLS

Seamless MPLS uses BGP-LU to create unified MPLS network:

┌─ Access ─┐   ┌─ Aggregation ─┐   ┌─ Core ─┐
AN ── AGN ── ABR ── CR ── ABR ── AGN ── AN


AN = Access Node
AGN = Aggregation Node
ABR = Area Border Router
CR = Core Router


BGP-LU provides end-to-end label path
No LDP needed in core

Benefits

  • Single label stack (not stacked LDP + BGP labels)
  • Scales to millions of prefixes
  • Consistent forwarding behavior

BGP-LU with VPN Services

L3VPN over BGP-LU

Terminal window
configure


# BGP-LU for transport
set protocols bgp address-family ipv4-labeled-unicast


# VPNv4 for customer routes
set protocols bgp address-family ipv4-vpn


# VPNv4 uses BGP-LU next-hop for label stack


commit

Label Stack

Outer label: BGP-LU label (transport)
Inner label: VPN label (service)


[VPN Label | BGP-LU Label | IP Packet]

BGP-LU Best Practices

1. Use Route Reflectors

Terminal window
# BGP-LU at scale needs route reflectors
# Same as regular BGP


set protocols bgp neighbor 10.255.0.100 remote-as 65001
set protocols bgp neighbor 10.255.0.100 address-family ipv4-labeled-unicast
set protocols bgp neighbor 10.255.0.100 update-source lo

2. Filter at Boundaries

Terminal window
# Don't accept labeled routes from customers
set policy prefix-list INFRA-ONLY rule 10 prefix 10.255.0.0/16 le 32
set policy prefix-list INFRA-ONLY rule 10 action permit


set policy route-map LU-IN rule 10 match ip address prefix-list INFRA-ONLY
set policy route-map LU-IN rule 10 action permit
set policy route-map LU-IN rule 20 action deny


set protocols bgp neighbor 192.0.2.1 address-family ipv4-labeled-unicast route-map import LU-IN

3. Consistent Label Allocation

Terminal window
# All routers should use consistent label allocation policy
# Usually per-prefix labeling for PE loopbacks

Troubleshooting BGP-LU

Labels Not Exchanged

Terminal window
# Check capability negotiation
show bgp neighbors 10.0.0.2


# Look for:
# IPv4 Labeled Unicast: advertised and received


# If not:
# - Check address-family configuration
# - Check both sides support labeled unicast

Wrong Label

Terminal window
# Show label for specific prefix
show bgp ipv4 labeled-unicast 10.255.0.2/32


# Verify label in MPLS table
show mpls table


# Check label is being used for forwarding

Path Not Using Labels

Terminal window
# Verify next-hop is reachable via MPLS
show bgp ipv4 labeled-unicast 10.255.0.2/32


# Check next-hop resolution
# BGP-LU requires next-hop to have label

Migration from LDP to BGP-LU

Parallel Operation

Terminal window
# Run both LDP and BGP-LU during migration
# LDP handles existing paths
# BGP-LU handles new paths


# Gradually shift traffic to BGP-LU
# Remove LDP when stable

Order of Operations

1. Enable BGP-LU on all routers
2. Verify BGP-LU paths working
3. Prefer BGP-LU over LDP (if needed)
4. Disable LDP
5. Clean up LDP configuration

The Lesson

BGP-LU replaces LDP in modern scalable designs.

LDP works within a single AS but:

  • Doesn’t cross AS boundaries
  • Requires separate protocol maintenance
  • Scales with IGP (limited)

BGP-LU advantages:

  • Works across AS boundaries
  • Uses existing BGP infrastructure
  • Scales with BGP (better)
  • Enables Seamless MPLS designs

When to use BGP-LU:

  • Multi-AS MPLS networks
  • Large-scale service provider networks
  • New MPLS deployments

VyOS BGP-LU support is functional for basic scenarios. For production SP networks, verify feature completeness in your VyOS version.