Label Distribution and Signalling Protocols (LDP, RSVP-TE)
What this domain tests
Label Distribution and Signalling Protocols examines how routers exchange label bindings. LDP distributes labels for IGP prefixes; RSVP-TE distributes labels while also reserving bandwidth and carrying an explicit path for traffic-engineering tunnels.
Core rules
- LDP discovers neighbors with UDP port 646 and forms sessions over TCP port 646. It binds a label to a FEC, commonly an IGP prefix, and advertises the binding with a Label Mapping message.
- LDP's default distribution mode is downstream unsolicited, and liberal label retention keeps bindings even when they are not currently best. This speeds convergence but uses more label space.
- RSVP-TE uses PATH messages downstream to carry the explicit route and reservation request, and RESV messages upstream to install reservation state and labels. State is soft state refreshed periodically.
- RSVP-TE can signal bandwidth, affinity, and explicit hops; LDP does not perform constraint-based path selection or reserve bandwidth.
Worked example
PE1 needs a 50 Mbps tunnel to PE2 avoiding a congested link. CSPF prunes links without 50 Mbps or with an excluded admin group, then computes a path. PE1 sends PATH with the explicit route and bandwidth. Each transit router forwards PATH, and PE2 returns RESV with a label. PE1 installs the tunnel and pushes the RSVP label for traffic steered into it. The reservation is made hop by hop, so the tunnel is not simply a label advertisement.
Traps
LDP hello and session both use port 646 but different transports. Do not say LDP reserves bandwidth. Do not confuse an RSVP session with a TE tunnel's IGP shortcut; the tunnel is unidirectional.