• Neighbor Discovery
    • Auto discovers neighbors on interfaces via ‘Hello’ message.
      • Hello source and destination UDP 646
      • ‘All Routers’′
    • 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.


  • 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’

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