3Com Switch 8800 Configuration Guide Chapter 35 MPLS Architecture35-635.3.3 Establishing LSPActually, the establishment of LSP refers to the process of binding FEC with the label,and then advertising this binding to the adjacent LSR on LSP. This process isimplemented through LDP, which regulates the message in interactive processing andmessage structure between LSRs as well as routing mode.I. LDP working processThrough sending Hello message periodically, an LSR finds its neighbor and thenestablish LDP session with the newly discovered adjacent LSR. By LDP session, theadjacent LSRs advertise such information as label switching mode, label space,session keepalive timer value to each other. LDP session is a TCP connection, whichneeds to be maintained through LDP message. In case there is not any other LDPmessage during the time period specified by the session keepalive timer value, andthen it is necessary to send session keepalive message to maintain the existence ofLDP session. Figure 35-4 illustrates the diagram of LDP label distribution.MPLS edge router (LER)LSP1MPLS core router (LSR)IngressEgressLDP sessionLSP2Label request messageLabel map messageA B CDEF GHMPLS edge router (LER)LSP1MPLS core router (LSR)IngressEgressLDP sessionLSP2Label request messageLabel map messageA B CDEF GHFigure 35-4 Label distribution processFor the label distribution mentioned previously, there are two modes: DoD and DU. Themain difference between these two modes is that the label mapping messages aredistributed actively or passively.In DoD mode, the label is distributed in this way: the upstream LSR sends label requestmessage (containing FEC descriptive information) to the downstream LSR, and thedownstream LSR distributes label for this FEC, and then it sends the bound label backto the upstream LSR through label map message. The time when the downstream LSRfeeds back the label map message depends on whether this LSR uses independentlabel control mode or sequential label control mode. When the sequential label controlmode is used by the downstream LSR, the label map message is sent back to itsupstream LSR if only it has received the label map message from its downstream LSR.And when the independent label control mode is used by the downstream LSR, then itwill send label map message to its upstream LSR immediately, no matter whether it has
