User Manual

80-8
Chapter 80 LDP
80.1 LDP Introduction
LDP protocol is used for label distribution in the MPLS label switching environment, and only applies to
networks capable of label switching. LDP, integrated with traditional routing algorithm, distribute labels,
advertise <label, FEC> map, create and maintain Label Forwarding Information Base and LSP, b y
transmitting various messages via TCP connections. LDP is used to distributing public network label in the
MPLS VPN environment.
LDP doesn’t create any route; instead, it obtains routes from the system, distributes labels for them and
advertises the labels to its upstream router. At the same time, for the FEC having a downstream, LDP will
receive a label from the downstream, take it as the outgoing label and create a label switched path, which
means to create an entry of switching the incoming label as the outgoing one. If the label distributed by the
downstream is 3, the LDP will create an entry of popping out the label.
LDP is defined in RFC3036; and its latest standard is RFC5036. It switches the map between labels and
routes via the TCP connection between peers. Two neighbor discovery modes are supported by LDP: the
basic mode (automatic discovery) and the extended mode (specified). The automatic discovery of peers is
implemented via the UDP multicast messages to all routers (224.0.0.2), using the port 646 in both TCP and
UDP messages.
The main process is as follows:
 Discover and maintain neighbors: after LDP is global enabled and interface enabled, it will send
multicast Hello messages on the specified interface (unless it disables the multicast-based neighbor
discovery) to advertise the network about its existence. The Hello messages will carry its
transmission address, the address for TCP connections. The adjacency will be created when
receiving Hello messages from other LSRs, and maintained by periodically sending Hello
messages.
 Establish and maintain sessions: LDP sessions are TCP-based; First, compare the transmission
address in the Hello message from the other end and that from this end, set the one with bigger
value as ACTIVE and the other PASSIVE. The ACTIVE router will initiate a connect request to
establish a TCP connection (to avoid the similar connection conflict problems suffered by BGP
neighbors). Once the TCP connection is established, the two parts will send initialization messages
to negotiate session parameters. A session will be established once the negotiation succeeds. After
that, the two neighbors will send the local interface address list and label information to each other.
To hold the connection when there is no data, KEEPALIVE messages will be sent.
 Create and maintain LSP: a session is necessary for each pair of LSR peers to switch label
information, which create LSP by switching FEC and label binding messages.
 Cancel sessions: Without any message from the other end for a long time, LDP will disconnect the
session and notify the close of the session to the other end by sending a notification messages.
Please notice that, LDP won’t distribute labels for default routes, or BGP routes (unless explicitly specified).