1. Discovering LDP Neighbors Link to heading

Session Establishment

Step 1: LDP Discovery Link to heading

  • 2 methods of neighbor discovery:
    • Basic: discover directly connected LSR neighbors. An LSR sends out periodic Hellos as UDP (port 646) to multicast address 224.0.0.2.
    • Extended: discover non-directly connected neighbors. An LSR sends out unicast Targeted Hellos as UDP to the IP address of the targeted LSR. # mpls ldp neighbor 192.168.1.1 targeted ldp

Step 2: LDP Session Establishment Link to heading

  • After the exchange of Hellos, LDP session is established:
    • 2 LSRs elect ative and passive roles by comparing the source IP address.
    • TCP (port 646) is used to establish the session.
    • Negotiate parameters by using Initialization messages.

2. Frame-mode MPLS Link to heading

MPLS Label Space: Link to heading

  • Per-Interface: Labels are assigned per FEC per interface => Packets are forwarded based on both the incoming label and interface.
  • Per-platform: Labels assigned per FEC are the same on any interface => Packets are forwarded based on incoming label only.

MPLS Label distribution modes: Link to heading

  • Downstream on-demand: each LSR sends a label binding request to its downstream router only (ex: RSVP for TE).
  • Unsolicited Downstream: each LSR automatically distributes its label bindings to all peers without a request message.

MPLS Label retention modes: Link to heading

  • Liberal: every label received from a neighbor is retained in LIB => Advantage: quick reaction to topology changes.
  • Conservative: label will only be retained if the sending LSR is the next hop downstream router for the specific FEC => Advantage: more secure than liberal mode.

MPLS Label Control modes: Link to heading

  • Independent label distribution: An LSR assigns a local binding to a FEC as soon as it realizes its existence in the routing table, does not wait to receive any labels from other LSR.
  • Ordered label distribution: An LSR assigns a local binding to a FEC only if it recognizes it is the egress LSR for that FEC or if it receives a label binding from the next hop LSR.

Frame-mode MPLS parameters include: Link to heading

  • Per-platform label address space
  • Unsolicited downstream label distribution
  • Independent control for label allocation
  • Liberal label retention