PLANET XGS3-24042 - Manual (Page 660)

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incoming labels.
 LSR will map the labels of received packets to NHLFE;
 LSR will find the corresponding NHLFE in the LIB based on the label, replace it with the new label
and then forward the label packet.
80.1.4 LDP Session
There are four steps to establish a LDP session:
 Discover
 Establish and maintain the session
 Create LSP
 Cancel the session
Discover
At this step, the LSR will send Hello messages periodically to adjacent LSRs, notifying them about its
existence, in order to establish a session. In the basic discover mechanism, LSR will discover its LDP peers
automatically via this process without manual configuration. There are two discover mechanisms:
 Basic Discover Mechanism
The Basic Discover Mechanism is used to discover local LDP peers – LSRs directly connected via the link
layer, and create a local LDP session. In this mode, the LSR will send LDP Link Hello messages periodically
via UDP messages to the multicast address marked as “all routers in the subnet”.
LDP Link Hello messages carry the LDP ID of the interface and other related information. If the LSR receives
a LDP Hello Message at an interface, it means that there is a LDP peer at this interface (Link Layer).
 Extended Discover Mechanism
The extended discover mechanism is used to discover remote LDP peers – LSRs not directly connected via
the link layer, and created remote LDP sessions. In this mode, the LSR will send LDP Targeted Hello
messages periodically to the specified IP address via UDP messages.
LDP Targeted Hello messages carry the LDP ID of the interface and other related information. If the LSR
receives a LDP Targeted Message at an interface, it means that there is a LDP peer at Network Layer.
Establish and Maintain the Session
After discovering a LDP peer, LSR will began to establish the session in two steps:
 Establish the transmission layer connection, that is, a TCP connection between LSRs;
 Initiate the session between the LSRs, negotiate all concerning parameters, such as the LDP
version, the label advertisement mode, the timer value, the label space. After the negotiation
succeeds, the session is established between the LSRs.
The session will be maintained by Keepalive messages after established.
Create LSP
The process of creating LSP is mapping FEC and labels and advertising the maps to the adjacent LSRs along
the LSP, which is realized via LDP. Take DoD mode as the example, the main steps are as follows:
（1） When the network routes change, if an edge router finds out a new destination address in its route
table which belongs to none of the existing FECs, it needs to create a new FEC for this destination
address. The edge LSR determines the route for the FEC, initiates a label request message to its