PLANET XGS3-24042 - Manual (Page 661)

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downstream LSR, and specifies for which FEC this label request is.
（2） The downstream receiving the label request message will save this message, finds the
corresponding FEC next-hop according to the local route table and then sends a label request
message to its downstream.
（3） When the label request message reaches the destination router or the egress router of the MPLS
network, if the router has available labels, and judges the label request messages as legal, it will
distribute a label for the FEC, and send a label mapping message containing the label information to
its upstream;
（4） The LSR receiving the label mapping message will check the state of label request messages saved
locally. If there is a corresponding label request message of a FEC label mapping message in the
data base, LSR will distribute a label for the FEC, and add a new entry in its LFIB, and then send the
label mapping information to its upstream.
（5） When the ingress LSR receives a label mapping message, it also should add a corresponding entry
in its LFIB, and thus finish the creation of LSP.
Cancel the session
LDP maintains adjacency by checking Hello messages. It also maintains session by checking Keepalive
messages. If there is no Keepalive message received within a certain period of time, the LDP session will
close the connection.
Each LDP session can include one or more Hello adjacencies. LDP maintains Hello adjacency via periodical
Hello messages. If there is no LDP Discovery Hello message received within a certain period of time, the LDP
session will close the Hello connection. When closing the last Hello adjacency in the LDP session, LDP will
send notification messages, and close the transmission connection.
80.1.5 LDP Loop Detection
Creating LSP in the MPLS domain also needs to prevent loops. The LDP loop detection mechanism can
detect LSP loops and avoid them.
To detect loops in the MPLS domain, all LSRs should be enabling the loop detection. But when establishing
LDP sessions, the configurations of loop detection on the two parties don’t have to be the same.
There are two LDP loop detection modes:
The maximum hop count
It is the number of LSR passed by the label messages (including label mapping and label request). When LSR
transmits label information with the hop-count parameter, it will first increase the hop count by 1. When the
hop count reaches the configured maximum value, it means that a loop exists, and the LSP creation will fail. If
the hop count is 0, it means the hop count is unknown. The hop count of label messages is always 0. The
default maximum hop count is 255.
Path Vector
It is used to record the path information in label mapping or label request messages. At each hop, the LSP
checks whether its LSR ID is in the record. The following two conditions mean the existence of a loop and the
failure of the LSP creation.
 There is a record of this LSR in the path vector record;