Juniper BGP, Configuration Manual

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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 231.
LDP uses many messages to create LSPs, classified in the following four types:
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Discovery—To identify other LSRs
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Adjacency—To create, maintain, and end sessions between LSRs
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Label advertisement—To request, map, withdraw, and release labels
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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 established, LDP peers exchange keepalive messages that verify
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MPLS Label Distribution Protocols
JUNOSe 11.1.x BGP and MPLS Configuration Guide