•
LDP uses the basic discovery mechanism to discover directly connected LDP peers.
•
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 sender, it does so by
periodically sending targeted hellos to the initiating LSR. The exchange of targeted hellos
constitutes a hello adjacency for the two LSRs.
251Copyright © 2010, Juniper Networks, Inc.
Chapter 3: MPLS Overview
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