MPLS and LDP: Label Distribution and Label Switching Paths

MPLS and LDP: Label Distribution and Label Switching Paths

This guide covers RouterOS 7 MPLS fundamentals and the Label Distribution Protocol (LDP): enabling MPLS forwarding on interfaces, configuring LDP for automatic label exchange, verifying label-switched paths (LSPs), and reading the Label Forwarding Information Base (LFIB).

MPLS separates the IP routing decision from the per-hop forwarding action. Once a packet enters an MPLS domain, routers forward it by swapping short fixed-length labels rather than performing a full IP lookup at every hop. LDP automates the process of distributing those labels between routers so that end-to-end LSPs form automatically from the IGP routing table.

Overview

Label Operations

Every router in an MPLS domain performs one of three label operations on each packet:

Operation	Router Role	Action
Push	Ingress LER	Attach an MPLS label to an incoming IP packet, creating an MPLS packet.
Swap	Transit LSR	Replace the top label with a new label and forward to the next hop.
Pop	Egress LER	Remove the MPLS label and deliver the original IP packet.

Router Roles

• LER (Label Edge Router) — sits at the boundary between the IP and MPLS domains. The ingress LER pushes the first label; the egress LER pops the final label.
• LSR (Label Switch Router) — a core router that only swaps labels. It never examines the inner IP header.

Penultimate Hop Popping (PHP)

By default, RouterOS uses PHP: the router before the egress LER pops the label, forwarding a plain IP packet to the egress. This removes one lookup at the egress router. The label value 3 (implicit null) signals PHP to the penultimate hop.

Label Distribution Protocol (LDP)

LDP automates LSP creation from the IGP topology:

1. Hello discovery — LDP sends UDP hellos to the multicast address 224.0.0.202 on port 646 to discover directly connected LDP neighbors.
2. Session establishment — neighbors negotiate a TCP session on port 646 using their LSR-IDs (typically loopback addresses) as transport endpoints.
3. Label advertisement — each router assigns a local label to every IGP prefix it knows and advertises the binding to all LDP neighbors. Neighbors chain these bindings into end-to-end LSPs.

Prerequisites

• RouterOS 7.x (MPLS features are available by default; verify with /system/package/print)
• An IGP (OSPF or IS-IS) advertising loopback /32 addresses and transport subnets between all MPLS routers
• A stable loopback interface on each router to serve as the LDP LSR-ID and OSPF router-id
• IP connectivity between all LDP transport addresses before enabling LDP

Note

LDP builds LSPs for all IGP-known prefixes by default. Ensure your IGP is stable before enabling LDP to avoid excessive label churn.

Configuration

The examples below use a three-router linear topology matching the diagram above:

Router	Role	Loopback	Link to next-hop
R1	Ingress LER	1.1.1.1/32	ether1 — 10.0.0.0/30 → R2
R2	Transit LSR	2.2.2.2/32	ether1 — 10.0.0.0/30 → R1, ether2 — 10.0.0.4/30 → R3
R3	Egress LER	3.3.3.3/32	ether1 — 10.0.0.4/30 → R2

Step 1: Configure Loopback and IGP Underlay

Each router needs a loopback with a /32 address and an IGP advertising it. OSPF is used here; IS-IS works equally well.

R1:

# Create loopback bridge
/interface bridge add name=loopback

# Assign LSR-ID address
/ip address add address=1.1.1.1/32 interface=loopback

# Configure OSPF with router-id matching the loopback
/routing ospf instance add name=default router-id=1.1.1.1
/routing ospf area add name=backbone area-id=0.0.0.0 instance=default
/routing ospf interface-template add area=backbone interfaces=loopback passive
/routing ospf interface-template add area=backbone interfaces=ether1

R2:

/interface bridge add name=loopback
/ip address add address=2.2.2.2/32 interface=loopback
/routing ospf instance add name=default router-id=2.2.2.2
/routing ospf area add name=backbone area-id=0.0.0.0 instance=default
/routing ospf interface-template add area=backbone interfaces=loopback passive
/routing ospf interface-template add area=backbone interfaces=ether1,ether2

R3:

/interface bridge add name=loopback
/ip address add address=3.3.3.3/32 interface=loopback
/routing ospf instance add name=default router-id=3.3.3.3
/routing ospf area add name=backbone area-id=0.0.0.0 instance=default
/routing ospf interface-template add area=backbone interfaces=loopback passive
/routing ospf interface-template add area=backbone interfaces=ether1

Verify IGP reachability before proceeding:

# On R1 — should resolve to R3 loopback via OSPF
/ip route print where dst-address=3.3.3.3/32
/ping 3.3.3.3 count=3

Step 2: Enable MPLS on Interfaces

MPLS must be enabled on the physical interfaces that carry labeled traffic. Do not enable MPLS on the loopback itself.

All routers — enable MPLS on core-facing interfaces:

# R1
/mpls interface add interface=ether1

# R2
/mpls interface add interface=ether1
/mpls interface add interface=ether2

# R3
/mpls interface add interface=ether1

Verify:

/mpls interface print

Expected output shows each interface with MPLS flag set.

Step 3: Configure LDP

LDP requires a global LSR-ID (the loopback address) and per-interface enabling. LDP uses the LSR-ID as the transport address for TCP sessions, so it must match an address reachable via IGP.

All routers:

# R1 — set LSR-ID to loopback, enable LDP on core interface
/mpls ldp set lsr-id=1.1.1.1 enabled=yes
/mpls ldp interface add interface=ether1

# R2
/mpls ldp set lsr-id=2.2.2.2 enabled=yes
/mpls ldp interface add interface=ether1
/mpls ldp interface add interface=ether2

# R3
/mpls ldp set lsr-id=3.3.3.3 enabled=yes
/mpls ldp interface add interface=ether1

Step 4: Verify LDP Neighbor Sessions

LDP discovery starts within seconds of enabling LDP interfaces. Check for established sessions:

/mpls ldp neighbor print detail

A healthy neighbor entry shows state=established and lists the remote LSR-ID, local and remote label spaces, and session uptime. If only state=connecting is shown, the TCP session has not formed — check IGP reachability to the remote LSR-ID.

Verification

Check LDP Bindings

After sessions form, routers exchange label bindings for all IGP prefixes:

/mpls ldp binding print detail

Each entry shows:

• prefix — the destination prefix
• nexthop — the LDP next-hop
• local-label — the label this router advertises to others for this prefix
• remote-label — the label received from the upstream LDP neighbor

Read the Label Forwarding Information Base (LFIB)

The LFIB is the active forwarding table used for labeled packet switching:

/mpls forwarding-table print detail

Key fields:

Field	Meaning
in-label	Label value matched on incoming packets
out-label	Label pushed on outgoing packets (3 = PHP/implicit-null, 0 = explicit-null)
interface	Outgoing interface
nexthop	Next-hop IP address
operation	swap, pop, or push

Verify LSP End-to-End

From the ingress LER, confirm a full LSP exists to the egress loopback:

# On R1 — show binding for R3 loopback
/mpls ldp binding print where prefix=3.3.3.3/32

# Confirm LFIB has forwarding entry
/mpls forwarding-table print where nexthop~"10.0.0"

If R2 is performing PHP, traffic from R1 will carry a label all the way to R2, where R2 pops it and sends a plain IP packet to R3. This is correct and expected behaviour.

Check Global MPLS Settings

/mpls settings print

Review propagate-ttl (whether IP TTL is decremented through the MPLS domain) and allow-fast-path (hardware/fast-path label switching).

Practical Examples

Example 1: Full LSP Verification on a Three-Router Chain

After completing the configuration above, run these checks on each router:

R1 (Ingress LER):

# LDP neighbor to R2 should be established
/mpls ldp neighbor print

# Binding for 3.3.3.3/32 should show remote-label from R2
/mpls ldp binding print where prefix=3.3.3.3/32

# LFIB should push R2's advertised label for traffic to 3.3.3.3/32
/mpls forwarding-table print detail

R2 (Transit LSR):

# Two neighbors: R1 and R3
/mpls ldp neighbor print

# Bindings for both 1.1.1.1/32 and 3.3.3.3/32
/mpls ldp binding print

# LFIB shows swap or pop operations for each prefix
/mpls forwarding-table print detail

R3 (Egress LER):

# LDP neighbor to R2
/mpls ldp neighbor print

# Bindings — local-label for 3.3.3.3/32 should be 3 (implicit-null, triggering PHP at R2)
/mpls ldp binding print where prefix=3.3.3.3/32

Example 2: Targeted LDP Sessions

For applications like VPLS that require LDP sessions between non-adjacent routers, use targeted (unicast) hellos:

# On R1 — establish targeted LDP session to R3 loopback
/mpls ldp neighbor add address=3.3.3.3

Targeted sessions use the same TCP port 646 but send unicast hellos instead of multicast. Verify with:

/mpls ldp neighbor print detail where address=3.3.3.3

Troubleshooting

LDP Neighbors Not Forming

# 1. Verify MPLS is enabled on the interface
/mpls interface print detail

# 2. Verify LDP is enabled on the interface
/mpls ldp interface print detail

# 3. Check LDP global settings — lsr-id must be set
/mpls ldp print

# 4. Confirm IGP reachability to the remote LSR-ID
/ip route print where dst-address=<remote-lsr-id>/32
/ping <remote-lsr-id>

If /ping to the remote LSR-ID fails, fix the IGP underlay first. LDP TCP sessions cannot establish without routable transport addresses.

Labels Not Being Distributed

# Confirm session is established (not just discovered)
/mpls ldp neighbor print detail

# Check binding table — if empty, LDP session may be down
/mpls ldp binding print

# Verify OSPF is advertising all loopbacks and transport subnets
/routing ospf neighbor print
/ip route print where protocol=ospf

Traffic Not Being Label-Switched

# Confirm LFIB has entries for destination prefixes
/mpls forwarding-table print detail

# Check if fast-path is enabled (hardware switching)
/mpls settings print

# Trace label path
/tool traceroute <destination> use-dns=no

Common Issues

Symptom	Likely Cause	Resolution
state=connecting in neighbor table	No IP route to remote LSR-ID	Fix IGP; ensure loopbacks are advertised
Empty binding table after session up	LDP enabled=no globally	/mpls ldp set enabled=yes
LFIB missing entries	MPLS not enabled on interface	/mpls interface add interface=<iface>
PHP not occurring at penultimate hop	Remote router not advertising implicit-null	Check egress router LDP binding for label=3 on its own prefix
Labels assigned but traffic drops	MTU too small for MPLS overhead	Add 4 bytes per label to interface MTU; check /mpls settings

Command Reference

/mpls interface

Enables MPLS forwarding on a physical interface.

Parameter	Description
interface	Interface name

/mpls interface add interface=ether1
/mpls interface print detail
/mpls interface remove [find interface=ether1]

/mpls ldp

Global LDP settings.

Parameter	Description
enabled	Enable/disable LDP globally (yes/no)
lsr-id	Router LSR-ID, used as LDP transport address — typically loopback /32
transport-addresses	Override transport address per address-family

/mpls ldp set lsr-id=1.1.1.1 enabled=yes
/mpls ldp print

/mpls ldp interface

Enables LDP on a specific interface for neighbor discovery.

/mpls ldp interface add interface=ether1
/mpls ldp interface print detail

/mpls ldp neighbor

View discovered and established LDP neighbors. Use add address= for static/targeted neighbors.

/mpls ldp neighbor print detail
/mpls ldp neighbor add address=3.3.3.3

/mpls ldp binding

Label binding table — shows local and remote labels for each prefix.

/mpls ldp binding print detail
/mpls ldp binding print where prefix=10.0.0.0/24

/mpls forwarding-table

Active LFIB — the table used for label-based packet forwarding decisions.

/mpls forwarding-table print detail
/mpls forwarding-table print where in-label=100

/mpls settings

Global MPLS behaviour flags.

/mpls settings print
/mpls settings set propagate-ttl=yes

Related Information

Related Topics

• VPLS — Layer 2 VPN services built on top of MPLS LDP LSPs
• MPLS Traffic Engineering and RSVP-TE — TE tunnels, explicit paths, and bandwidth reservation with RSVP-TE
• MPLS-TE Explicit Paths — Detailed explicit path configuration and ERO notation
• MPLS-TE Hop Types — Strict and loose hop configuration for TE tunnels
• OSPF Configuration — IGP underlay required for LDP LSR-ID reachability

RFC References

• RFC 5036 — LDP Specification (core protocol)
• RFC 3031 — MPLS Architecture
• RFC 3032 — MPLS Label Stack Encoding
• RFC 3443 — TTL Processing in Multi-Protocol Label Switching (MPLS) Networks