Functions and features of the LDP Protocol

Many of my friends have asked about the content of the LDP protocol. Therefore, I think it is necessary to introduce this part in detail. Let's take a look at the basic structure and functions of the LDP protocol. Through the LDP protocol, LSR can directly map the routing information at the network layer to the switching path at the data link layer, and then establish the LSP at the network layer. The LSP can be established between two adjacent lsrs or end at the egress node of the network, so that labels can be used for all intermediate nodes in the network.

LDP protocol peer

LDP protocol peers refer to two lsrs that have LDP sessions and exchange tag/FEC ing relationships using the LDP protocol. The two LDP protocol peers can obtain the tag ing message of the peer through one LDP session at the same time, that is, the LDP protocol is bidirectional.

LDP session

LDP sessions are used to exchange tag ing, release, and other messages between LSR. LDP sessions can be divided into two types: Local LDP sessions): The two lsrs that establish sessions are directly connected; Remote LDP sessions ): the two lsrs used to establish a session are not directly connected;

LDP message

LDP mainly uses four types of messages: discovering Discovery) messages: used to advertise and maintain the presence of LSR in the network; Session) messages: used to establish, maintain, and terminate Session connections between LDP peers; advertise Advertisement) messages: used to create, change, and delete tags-FEC binding; Comware V3 operation manual MPLS ). Notification) message: used to provide the message and Error Notification of the token.

Tag space and LDP identifier

The range of tags allocated between LDP peers is called tag space. You can specify a tag space for each LSR interface, or use a tag space for the entire LSR. The LDP protocol is used to identify the tag space range of a specific LSR. It is a six-byte value in the following format: the four-byte IP address is the IP address of the LSR, the tag space number occupies two bytes.

LDP Protocol Working Process

On an LSP, the adjacent LSR is called the upstream LSR and the downstream LSR along the data transmission direction. For example, in LSP1, lsr B is the upstream LSR of LSR C. The tag distribution process consists of downstream on-demand tag distribution DoD and downstream independent tag distribution DU. The main difference between them is whether the tag ing is published by upstream requests or downstream active releases. The label distribution process of the two modes is described as follows:

(1) DoDdownstream-on-demand) Mode

The upstream LSR sends a tag request message to the downstream LSR, which contains the description of the FEC. The downstream LSR assigns a tag to the FEC and sends the bound tag to the upstream LSR through the tag ing message. When the downstream LSR returns the tag ing message, it depends on whether the LSR adopts the independent tag control mode or the ordered tag control mode. When the ordered tag control mode is adopted, only the label ing message Comware V3 operation manual MPLS is received from its downstream return, and the tag ing message is sent to its upstream. When the independent tag control mode is adopted, no matter whether or not it receives the tag ing message returned by its downstream, it immediately sends a tag ing message to its upstream. Upstream LSR generally selects downstream LSR based on the information in its route table. In Figure 1-4, LSR along LSP1 adopts the ordered label control mode, while lsr f on LSP2 adopts the independent label control mode.

(2) DUdownstream unsolicited) Mode

After the LDP protocol session is established successfully, the downstream LSR actively publishes a tag ing message to the upstream LSR. The upstream LSR stores the tag ing information and processes the received tag ing information based on the route table information.