Path Computation Element Communication Protocol (PCEP) is defined as a protocol that can work between two devices, one forwarding using Traffic Engineering (Router) while the other performing all of the computations for determining the traffic engineering paths (SDN Controller). It is defined in Request For Comment (RFC) 4655 and as per RFC device running a Traffic-Engineering (TE) protocol known as Path Computation Client (PCC). The device which does all the computation is referred to as a Path Computation Element (PCE), and naturally the protocol between the PCE and PCC takes up the name PCEP.

Traditionally, the routers perform their own computations and exchange information between each other for that purpose. In the PCEP model, the router (acting as a PCC) does the forwarding and label imposition, disposition, etc. but it leaves the entire computation and path decision-making process to the PCE. If there are multiple PCEs working collectively, PCEP may also be used as a communication protocol between them. To learn the Link State Database (LSDB) information from the network, the PCE device could establish a passive IGP relationship with the devices and share the entire traffic engineering database. This approach is limited to single area only as one area doesn’t have the link state information of other area.

PCEP – SDN Control Plane Protocol