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NE20E-S V800R010C10SPC500 Feature Description - MPLS 01

This is NE20E-S V800R010C10SPC500 Feature Description - MPLS
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Basic Concepts

Basic Concepts

The MPLS architecture consists of multiple label distribution protocols, in which LDP is widely used.

LDP defines messages in the label distribution process and procedures for processing the messages. Label switching routers (LSRs) obtain information about incoming labels, next-hop nodes, and outgoing labels for specified FECs based on the local forwarding table. LSRs use the information to establish LSPs.

For detailed information about LDP, see relevant standards (LDP Specification).

LDP Adjacency

When an LSR receives a Hello message from a peer, the LSR establishes an LDP adjacency with the peer may exist. An LDP adjacency maintains a peer relationship between the two LSRs. There are two types of LDP adjacencies:
  • Local adjacency: established by exchanging Link Hello messages between two LSRs.

  • Remote adjacency: established by exchanging Target Hello messages between two LSRs.

LDP Peers

Two LDP peers set up LDP sessions and exchange Label Mapping messages over the session so that they establish an LSP.

LDP Session

An LDP session between LSRs helps them exchange messages, such as Label Mapping messages and Label Release messages. LDP sessions are classified into the following types:

  • Local LDP session: set up over a local adjacency. The two LSRs, one on each end of the local LDP session, are directly connected.

  • Remote LDP session: set up over a remote adjacency. The two LSRs, one on each end of the remote LDP session, can be either directly or indirectly connected.

The local and remote LDP sessions can be set up simultaneously.

Relationships Between LDP Adjacencies, Peers, and Sessions

LDP maintains the presence of a peer through an adjacency, and the type of peer depends on the type of neighbor that maintains it. A peer can be maintained by multiple neighbors, and if it is maintained by both the local adjacency and the remote adjacency, the peer type is a distant coexistence peer. Only a peer can establish an LDP session.

LDP Messages

Two LSRs exchange the following messages:

  • Discovery message: used to notify or maintain the presence of an LSR on an MPLS network.

  • Session message: used to establish, maintain, or terminate an LDP session between LDP peers.

  • Advertisement message: used to create, modify, or delete a mapping between a specific FEC and label.

  • Notification message: used to provide advisory information or error information.

LDP transmits Discovery messages using the User Datagram Protocol (UDP) and transmits Session, Advertisement, and Notification messages using the Transmission Control Protocol (TCP).

Label Space and LDP Identifier

  • Label space

    A label space defines a range of labels allocated between LDP peers. The NE20E only supports per-platform label space. All interfaces on an LSR share a single label space.

  • LDP identifier

    An LDP identifier identifies a label space used by a specified LSR. An LDP identifier consists of 6 bytes including a 4-byte LSR ID and a 2-byte label space. An LDP identifier is in the format of <LSR ID>:<Label space ID>.

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Updated: 2019-01-03

Document ID: EDOC1100055123

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