Before discussing the CE-LDP, lets explaint “constraint-based routing” term.
“Contraint-based Routing” is the routing which in which the routing process is done according to some constraints. Both QoS-based routing and policy-based routing belong to Contraint-based Routing. As you know these routing include QoS constraints or policy constraints.
CR-LDP (constraint-based LDP) contains extensions for LDP. And with these extensions, Contraint-based Routing is supported on LDP. In other words, Contraint-based Routing brings Traffic Engineering(TE) mechanism to the LDP.
The additional TLV parameters are new for CR-LDP:
Value Parameter ----- --------- 821 LSPID 822 ResCls 503 Optical Session Parameters 800 Explicit Route 801-804 ER-Hop TLVS 810 Traffic Parameters 820 Preemption 823 Route Pinning 910 Optical Interface Type 920 Optical Trail Desc 930 Optical Label 940 Lambada Set