Dynamic routing

LDP

Also called label distribution protocol, UDP 646, TCP 646, label binding

Hands out MPLS labels: each router picks a label for every route it has and tells its neighbours.

Routers find each other with hellos to 224.0.0.2 on UDP port 646, then exchange labels over a TCP session on port 646. A router pushes or swaps to the label its next hop picked, so labelled paths follow the routes OSPF (or another protocol) found. A link without an LDP session breaks the path: the routers on it fall back to plain IP.

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Learn it on real machines.