3Com Router 3000 Ethernet FamilyConfiguration Guide Chapter 1 MPLS Architecture3Com Corporation1-10sends a label mapping message to the upstream LSR with the allocated labelinformation included;4) The upstream LSR compares the received label mapping message with its labeldatabase, allocates the matched label to the FEC, adds the map to its labelforwarding table, and then sends the label mapping message to its upstream LSR;5) When the Ingress ISR receives the label mapping message, it adds the map to itslabel forwarding table. In this way, an LSP is set up and the corresponding FECdata packet can be forwarded based on its label.IV. Session TerminationThe LDP checks the session integrity depending on the LDP PDU transmitted in thesession connection.The LSR sets up a living timer for each session and refreshes the timer after receivingan LDP PDU. If the timer expires before the reception of an LDP PDU, the LSRconsiders the session interrupted and tears down the corresponding connection on thetransport layer to terminate the session.1.4.4 LDP Loop DetectionIt is necessary to prevent path loop from happening while establishing LSP in the MPLSdomain. The LSP loop detection mechanism can detect such path loop and avoidmessage loop occurring such as the label request message.To avoid the LSP path loop, two methods can be used:I. Maximum Hop CountThe maximum hop count method is to contain the hop-count information in themessage bound with the forwarding label. This value is added by one for each hop.When the value exceeds the specified maximum value, it is considered that a loophappens, and the process for establishing LSP is terminated.II. Path VectorThe path vector method is to record the path information in the message bound with theforwarding label. For every hop, the corresponding router checks if its ID is contained inthis record. If not, the router adds its ID into the record; and if so, it indicates that a loophappens and the process for establishing LSP is terminated.1.4.5 Constrain-based Routing LDPMPLS also supports Constrain-based Routing LDP mechanism (CR-LDP). Theso-called CR-LDP refers to that, while originating the establishment of LSP, the ingressnode adds some constrain information for LSP routing in label request message. Tworouting approaches are available: strict explicit routing, where the constraint
