Chapter 3: Label Distribution Protocol
Master the Label Distribution Protocol (LDP), the standard protocol for distributing labels in MPLS networks, including neighbor discovery, session establishment, and label advertisement mechanisms.
LDP Overview
Label Distribution Protocol (LDP) is the IETF standard protocol for distributing labels between LSRs. It establishes label-switched paths automatically based on the IP routing table.
LDP Characteristics
LDP follows the IP routing table, creating LSPs along the shortest path. It's a hop-by-hop protocol that distributes labels for IP prefixes automatically.
LDP Advantages
- Automatic label distribution
- Simple configuration
- Follows IP routing
- Loop-free by design
- Standards-based
LDP Limitations
- No traffic engineering
- Follows IGP paths only
- No QoS guarantees
- Limited path control
- TCP-based (overhead)
Neighbor Discovery
LDP uses a two-step process for neighbor discovery: basic discovery for directly connected neighbors and extended discovery for non-adjacent neighbors.
| Discovery Type | Method | Port | Scope | Use Case |
|---|---|---|---|---|
| Basic Discovery | UDP Hello (Multicast) | 646 | Local subnet | Directly connected LSRs |
| Extended Discovery | UDP Hello (Unicast) | 646 | Network-wide | Non-adjacent LSRs |
Basic Discovery Process
- LSRs send periodic Hello messages
- Multicast to 224.0.0.2 (All Routers)
- Contains LDP Identifier (Router ID + Label Space)
- Neighbors respond with their own Hello
- Adjacency established when both send Hellos
Hello Message Contents
- LDP Identifier: Router ID + Label Space
- Hold Time: Neighbor timeout value
- Transport Address: TCP session address
- Configuration Number: Session parameters
Session Establishment
After neighbor discovery, LDP establishes TCP sessions for reliable label distribution. The session establishment follows a specific sequence of message exchanges.
Session Establishment Steps
1. TCP Connection → 2. Initialization → 3. Parameter Negotiation → 4. Session Active
1. TCP Connection
TCP session on port 646. Higher LDP ID initiates connection.
2. Initialization
Exchange Initialization messages with session parameters.
3. Parameter Negotiation
Negotiate protocol version, timers, and capabilities.
4. Session Active
Session established, ready for label distribution.
| Parameter | Description | Default Value | Negotiation |
|---|---|---|---|
| Protocol Version | LDP protocol version | 1 | Use lower version |
| Keepalive Time | Session keepalive interval | 60 seconds | Use smaller value |
| Label Distribution Method | Downstream-on-demand or unsolicited | Unsolicited | Must match |
| Loop Detection | Path vector or hop count | Disabled | Must match |
Label Advertisement
Once the LDP session is established, LSRs advertise labels for FECs to their neighbors. LDP supports different advertisement modes and label retention modes.
Downstream Unsolicited (DU)
Downstream LSR advertises labels without being asked.
- Default mode
- Faster convergence
- More memory usage
- Immediate availability
Downstream on Demand (DoD)
Upstream LSR requests labels from downstream neighbors.
- Conservative approach
- Less memory usage
- Slower convergence
- Request/response overhead
Liberal Label Retention
LSR keeps all received labels, even from non-next-hop neighbors.
- Fast rerouting capability
- Quick convergence on topology changes
- Higher memory consumption
Conservative Label Retention
LSR only keeps labels from next-hop neighbors for each FEC.
- Lower memory usage
- Simpler management
- Slower recovery on failures
Targeted LDP
Targeted LDP (tLDP) allows LDP sessions between non-adjacent LSRs, enabling MPLS services across multiple hops and supporting advanced MPLS applications.
Targeted LDP Use Cases
tLDP is essential for L3VPNs, L2VPNs, and any MPLS service requiring label distribution between non-adjacent routers.
| Feature | Basic LDP | Targeted LDP | Use Case |
|---|---|---|---|
| Neighbor Relationship | Adjacent only | Any reachable LSR | PE-PE sessions |
| Hello Messages | Multicast | Unicast | Remote neighbors |
| Configuration | Automatic | Manual configuration | Specific peer targeting |
| Transport | Interface IP | Loopback IP | Stable addressing |
L3VPN Applications
- PE-PE label exchange
- VPN label distribution
- Inter-AS VPNs
- Carrier's Carrier
L2VPN Applications
- Pseudowire signaling
- VPLS mesh setup
- L2 circuit provisioning
- Metro Ethernet services
Traffic Engineering
- TE tunnel tail labels
- Fast Reroute backup
- Explicit path support
- Bandwidth reservation
Next Steps
Now that you understand LDP operations, continue to Chapter 4: MPLS VPN Architecture to learn how MPLS enables advanced VPN services.