Planet XGS3-24042 User Manual download pdf (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;

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Planet XGS3-24042 User Manual Page 661

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