Ldp Loop Detection; Constrain-Based Routing Ldp - 3Com 3C13636 Configuration Manual

3Com Router 3000 Ethernet Family
Configuration Guide
sends a label mapping message to the upstream LSR with the allocated label
information included;
4) The upstream LSR compares the received label mapping message with its label
database, allocates the matched label to the FEC, adds the map to its label
forwarding 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 its
label forwarding table. In this way, an LSP is set up and the corresponding FEC
data packet can be forwarded based on its label.
IV. Session Termination
The LDP checks the session integrity depending on the LDP PDU transmitted in the
session connection.
The LSR sets up a living timer for each session and refreshes the timer after receiving
an LDP PDU. If the timer expires before the reception of an LDP PDU, the LSR
considers the session interrupted and tears down the corresponding connection on the
transport layer to terminate the session.

1.4.4 LDP Loop Detection

It is necessary to prevent path loop from happening while establishing LSP in the MPLS
domain. The LSP loop detection mechanism can detect such path loop and avoid
message loop occurring such as the label request message.
To avoid the LSP path loop, two methods can be used:
I. Maximum Hop Count
The maximum hop count method is to contain the hop-count information in the
message 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 loop
happens, and the process for establishing LSP is terminated.
II. Path Vector
The path vector method is to record the path information in the message bound with the
forwarding label. For every hop, the corresponding router checks if its ID is contained in
this record. If not, the router adds its ID into the record; and if so, it indicates that a loop
happens and the process for establishing LSP is terminated.

1.4.5 Constrain-based Routing LDP

MPLS also supports Constrain-based Routing LDP mechanism (CR-LDP). The
so-called CR-LDP refers to that, while originating the establishment of LSP, the ingress
node adds some constrain information for LSP routing in label request message. Two
routing approaches are available: strict explicit routing, where the constraint
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Chapter 1 MPLS Architecture