Size LDP sessions, label bindings and hello load across an MPLS core using liberal label retention.
LDP discovers neighbours with UDP hellos then builds a TCP session per pair, and by default keeps every label binding it hears. That liberal retention is what makes reconvergence fast — the label for the new next hop is already in memory — at the cost of memory scaling with FECs times neighbours. LDP memory and session count are what decide whether an MPLS core reconverges in milliseconds or seconds, and advertising more than the core loopbacks into LDP is the most common way to multiply bindings for no benefit.
LDP Session
Sessions = LSRs × neighbours ÷ 2, since a session is shared; with liberal retention each LSR stores one label binding per FEC per neighbour.
Binding and hello load
Bindings per LSR = FECs × neighbours; hello load = 2 × sessions ÷ hello interval.
Sessions = LSRs × neighbours ÷ 2, since a session is shared; with liberal retention each LSR stores one label binding per FEC per neighbour. LDP discovers neighbours with UDP hellos then builds a TCP session per pair, and by default keeps every label binding it hears. That liberal retention is what makes reconvergence fast — the label for the new next hop is already in memory — at the cost of memory scaling with FECs times neighbours.
LDP memory and session count are what decide whether an MPLS core reconverges in milliseconds or seconds, and advertising more than the core loopbacks into LDP is the most common way to multiply bindings for no benefit.
This calculator takes 5 inputs: LSRs in the core, LDP neighbours per LSR, FECs (loopback prefixes) advertised, Link hello interval, Hello hold time. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.
No. Filter LDP to core loopback /32s only. Every extra FEC multiplies bindings across every neighbour, and customer or aggregate prefixes never need a transport label — the label that matters is the one for the egress PE loopback.