LDP Overview 1323DU modeIn DU mode, a downstream LSR advertises label binding information to itsupstream LSR unsolicitedly after the LDP session is established, while the upstreamLSR keeps the label binding information and processes the information based onits routing table information.Fundamental Operationof LDPLDP goes through four phases in operation: discovery, session establishment andmaintenance, LSP establishment and maintenance, and session termination.DiscoveryIn this phase, an LSR who wants to establish a session sends Hello messages to itsneighboring LSRs periodically, announcing its presence. This way, LSRs canautomatically find their peers without manual configuration.LDP provides two discovery mechanisms:■ Basic discovery mechanismThe basic discovery mechanism is used to discover local LDP peers, that is, LSRsdirectly connected at link layer, and to further establish local LDP sessions.Using this mechanism, an LSR periodically sends LDP link Hellos as UDP packetsout an interface to the multicast address known as “all routers on this subnet”. AnLDP link Hello message carries information about the LDP identifier of a giveninterface and some other information. Receipt of an LDP link Hello message on aninterface indicates that a potential LDP peer is connected to the interface at linklayer.■ Extended discovery mechanismThe extended discovery mechanism is used to discover remote LDP peers, that is,LSRs not directly connected at link layer, and to further establish remote LDPsessions.Using this mechanism, an LSR periodically sends LDP targeted Hellos as UDPpackets to a given IP address.An LDP targeted Hello message carries information about the LDP identifier of agiven LSR and some other information. Receipt of an LDP targeted Hello messageon an LSR indicates that a potential LDP peer is connected to the LSR at networklayer.At the end of the discovery phase, Hello adjacency is established between LSRs,and LDP is ready to initiate session establishment.Session establishment and maintenanceIn this phase, LSRs pass through two steps to establish sessions between them:1 Establishing transport layer connections (that is, TCP connections) between them.2 Initializing sessions and negotiating session parameters such as the LDP version,label distribution mode, timers, and label spaces.
