Label Distribution Protocol (LDP)

• Neighbor Discovery
  • Auto discovers neighbors on interfaces via ‘Hello’ message.
    • Hello source and destination UDP 646
    • ‘All Routers’ 224.0.0.2′
  • Hello includes IPv4 Transport address.
    • Address to use for the TCP session.
    • Defaults to the LDP Router-ID
• Forming adjacency
  • LDP sessions are formed reliably over TCP
  • Unicast between transport addresses
  • TCP port 646
  • Implies peers must have routes to each other’s transport addresses.
  • Loopback
• Advertising Labels
  • Once LDP is established, label is advertised for Forwarding Equivalency Class (FEC)
    • Label to IPv4 prefix mapping.
  • Label distribution can be implicit or explicit.
    • Unsolicited Downstream vs. Downstream on demand.
    • Depends on implementation and config options.
  • Labels could be advertised for some or all routes
    • Cisco default all IGP routes.
    • Really only /32 Loopback matters.

Configuration:

• Enable CEF
  • Cisco always on default.
• Agree on label protocol
  • ‘mpls label protocol’
  • LDP by default.
• Recommended to define Router-ID
  • ‘mpls ldp router-id’
• Enable LDP
  • Interface ‘mpls ip’
  • IGP process ‘mpls ldp autoconfig’
• LDP Verification
  • ‘show mpls interfaces’
• Verify LDP sessions
  • ‘show mpls ldp neighbor‘
• Verify FIB
  • ‘show mpls forwarding-table’
• Troubleshooting adjacencies
  • ‘debug mpls ldp transport events’