LDP Overview1321nWhen an MPLS echo request reaches the egress, the destination address in the IPheader is set to an address on 127.0.0.0/8 (loopback address of the LSR) and theTTL is set to 1 so as to prevent further forwarding of the request.LDP OverviewLDP Basic ConceptsAn LDP dictates the messages to be used in label distribution and the relatedprocesses.Using LDP, LSRs can map network layer routing information to data layer switchingpaths directly and further establish LSPs. LSPs can be established between bothneighboring LSRs and LSRs that are not directly connected, making label switchingpossible at all transit nodes on the network.nFor detailed description about LDP, refer to RFC 2036 “LDP Specification”.LDP peerTwo LSRs with an LDP session established between them and using LDP toexchange label to FEC bindings are called LDP peers, each of which obtains thelabel to FEC bindings of its peer over the LDP session between them.LDP sessionLDP sessions are used to exchange messages for label binding and releasing.LDP sessions come in two categories:■Local LDP session: Established between two directly connected LSRs.■Remote LDP session: Established between two indirectly connected LSRs.LDP message typeThere are four types of LDP messages:■Discovery message: Used to declare and maintain the presence of an LSR on anetwork.■Session message: Used to establish, maintain, and terminate sessions betweenLDP peers.■Advertisement message: Used to create, alter, or remove label to FEC bindings.■Notification message: Used to provide advisory information and signal errors.For reliable transport of LDP messages, TCP is used for LDP session messages,advertisement messages, and notification messages, while UDP is used only fordiscovery messages.Label space and LDP identifierA scope of labels that can be assigned to LDP peers is called a label space. A labelspace can be per interface or per platform. A per interface label space isinterface-specific, while a per platform label space is for an entire LSR.An LDP identifier is used to identify an LSR label space. It is a six-byte numericalvalue in the format of :
