engineering (TE) or quality of service (QoS) capabilities, but they also support best-effort
LSPs.
LDP Messages and Sessions
LDP creates reliable sessions by running over TCP. You do not have to explicitly configure
LDP peers, because each LSR actively discovers all other LSRs to which it is directly
connected. LDP is a hard-state protocol, meaning that after the LSP is established, it is
assumed to remain in place until it has been explicitly torn down. This is in contrast to
RSVP-TE, which is a soft-state protocol. See “RSVP-TE Messages and Sessions” on
page 240.
LDP uses many messages to create LSPs, classified in the following four types:
•
Discovery—To identify other LSRs
•
Adjacency—To create, maintain, and end sessions between LSRs
•
Label advertisement—To request, map, withdraw, and release labels
•
Notification—To provide advisory and error information
Unlike the other LDP messages, the discovery process runs over UDP. Each LSR
periodically broadcasts a link hello message to the well-known UDP port, 646. Each LSR
listens on this port for link hello messages from other LSRs. In this manner, each LSR
learns about all other LSRs to which it is directly connected, creating link hello adjacencies.
When an LSR learns about another LSR, it establishes a TCP connection to the peer on
well-known TCP port 646 and creates an LDP session on top of the TCP connection.
A transport address for the local peer is advertised in LDP discovery hello messages.
Interfaces that use the platform label space default to the LSR router ID for the transport
address. You can use the mpls ldp discovery transport-address command to specify
an arbitrary IP address as the transport address.
LDP can also discover peers that are not directly connected if you provide the LSR with
the IP address of one or more peers by means of an access list. The LSR sends targeted
hello messages to UDP port 646 on each remote peer. If the targeted peer responds with
a targeted hello message to the initiator, a targeted hello adjacency is created and session
establishment can proceed.
In certain cases, a targeted hello adjacency to directly connected peers might be useful.
If an LSR receives both a link hello message and a targeted hello message from the same
initiator, only a single LDP session is established between the LSRs.
By default, because all LSRs listen on the well-known port, they all attempt to create a
session with the originator. You can use the mpls ldp link-hello disable command to
suppress the transmission of link hello messages. Thereafter, sessions are formed only
with peers contacted with targeted hello messages.
The LDP peers exchange session initialization messages that include timer values and
graceful-restart parameters. An LSR responds with a keepalive message if the values in
the initialization message are acceptable. If any value is not acceptable, the LSR responds
instead with an error notification message, terminating the session. After a session is
239Copyright © 2010, Juniper Networks, Inc.
Chapter 3: MPLS Overview
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