LDP LSRs discover each other by sending multicast ???hello??? messages to UDP port
646 over their physical links. It is also possible to run LDP between nonadjacent LSRs
by using ???extended discovery??? procedures where each LSR is configured to send unicast
???hello??? messages to the IP address of the other (again using UDP port 646). This
is known as targeted LDP and has applications in MPLS scaling and in pseudowire
signalling (discussed in the sections on MPLS scaling and establishing pseudowires
below). Once two LSRs have discovered each other, they establish an LDP session
over which labels can be advertised. The LDP session runs over TCP using port 646
and is typically established between loopback IP addresses on the two LSRs (these are
addresses that are internal to the node and are reachable if any of the nodes??™ physical
interfaces are up). Note that two LSRs will generally only establish one LDP session to
each other, even if LDP hellos have discovered multiple physical links between them.
As mentioned previously, LDP creates LSPs that follow the IGP shortest path. Where
there are multiple equal cost paths (multiple equal-cost links to the same next-hop
router and/or multiple next-hop routers with equal cost), packets will generally be
load-shared across those paths using an Equal-Cost Multi-Path (ECMP) algorithm.
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