Juniper JUNOSE, Configuration Manual

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LDP uses the basic discovery mechanism to discover directly connected LDP
peers.
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LDP uses the extended discovery mechanism to discover peers that are not
directly connected.
LDP Basic Discovery Mechanism
To discover directly connected peers, LSRs periodically send out LDP link hellos on
the interface. The link hellos are contained in UDP packets that are addressed to the
well-known LDP discovery port, 646. The destination address for the ports is
224.0.0.2. Using this port and address ensures that the hellos are sent to all routers
on the interface’s subnet.
The link hello includes the LDP identifier for the label space that the LSR intends to
use for the interface. In the JUNOSe implementation, this is always the platform label
space, so the LDP identifier specifies the LSR ID and a value of 0 for the label space.
The link hello also includes other information, such as the hello hold time configured
on the interface. The hello hold time specifies how long an LSR maintains a record
of hellos received from potential peers.
When an LSR receives a link hello, it identifies the sending LSR as a potential LDP
peer on that interface. The LSRs form a hello adjacency to keep track of each other.
The basic discovery mechanism is enabled by default when you enable LDP on an
interface. You can configure the link hellos in the LDP profile with the
hello hold-time
and hello interval commands. You can configure a transport IP address to be globally
included in link hellos with the mpls ldp discovery transport-address command.
LDP Extended Discovery Mechanism
To discover LDP peers that are not directly connected, LSRs periodically send out
LDP targeted hellos to potential peers. The targeted hellos are contained in UDP
packets that are addressed to the well-known LDP discovery port, 646. The destination
address for the ports is a specific targeted address. LDP sends targeted hellos when
you configure one or more IP addresses in a targeted-hello send list. In a layer 2
Martini circuit, targeted hellos are automatically sent to the remote PE neighbor (the
base tunnel endpoint). See
Configuring Layer 2 Services over MPLS for information
about layer 2 circuits.
The targeted hello includes the LDP identifier for the label space that the LSR intends
to use. In the JUNOSe implementation, this is always the platform label space, so
the LDP identifier specifies the LSR ID and a value of 0 for the label space. The
targeted hello also includes other information, such as the targeted-hello hold time,
which is configured globally. The targeted-hello hold time configures how long an
LSR waits for another targeted hello from its peer before declaring the adjacency to
be down.
Unlike basic discovery, where hellos are sent by all LSRs, extended discovery is
initiated by one LSR that targets a specific LSR. The initiating LSR periodically sends
targeted hellos to the targeted LSR. The targeted LSR then determines whether to
respond to the targeted hello or to ignore it. If the targeted LSR responds to the
LDP Discovery Mechanisms
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Chapter 2: MPLS Overview