I found the answer in RFC 3036 ... They're targeted.
3036
2.4. LDP Discovery
LDP discovery is a mechanism that enables an LSR to discover
potential LDP peers. Discovery makes it unnecessary to explicitly
configure an LSR's label switching peers.
There are two variants of the discovery mechanism:
- A basic discovery mechanism used to discover LSR neighbors that
are directly connected at the link level.
- An extended discovery mechanism used to locate LSRs that are
not directly connected at the link level.
2.4.1. Basic Discovery Mechanism
To engage in LDP Basic Discovery on an interface an LSR periodically
sends LDP Link Hellos out the interface. LDP Link Hellos are sent as
UDP packets addressed to the well-known LDP discovery port for the
"all routers on this subnet" group multicast address.
An LDP Link Hello sent by an LSR carries the LDP Identifier for the
label space the LSR intends to use for the interface and possibly
additional information.
Receipt of an LDP Link Hello on an interface identifies a "Hello
adjacency" with a potential LDP peer reachable at the link level on
the interface as well as the label space the peer intends to use for
the interface.
2.4.2. Extended Discovery Mechanism
LDP sessions between non-directly connected LSRs are supported by LDP
Extended Discovery.
To engage in LDP Extended Discovery an LSR periodically sends LDP
Targeted Hellos to a specific address. LDP Targeted Hellos are sent
as UDP packets addressed to the well-known LDP discovery port at the
specific address.
An LDP Targeted Hello sent by an LSR carries the LDP Identifier for
the label space the LSR intends to use and possibly additional
optional information.
Extended Discovery differs from Basic Discovery in the following
ways:
- A Targeted Hello is sent to a specific address rather than to
the "all routers" group multicast address for the outgoing
interface.
- Unlike Basic Discovery, which is symmetric, Extended Discovery
is asymmetric.
One LSR initiates Extended Discovery with another targeted LSR,
and the targeted LSR decides whether to respond to or ignore
the Targeted Hello. A targeted LSR that chooses to respond
does so by periodically sending Targeted Hellos to the
initiating LSR.
Receipt of an LDP Targeted Hello identifies a "Hello adjacency" with
a potential LDP peer reachable at the network level and the label
space the peer intends to use.
> -----Original Message-----
> From: Wessels, Tyler
> Sent: Monday, December 31, 2001 2:14 PM
> To: 'juniper-nsp@puck.nether.net'
> Subject: Junos 5.0 LDP Discovery Messages
>
> All,
> LDP discovery messages are sent as a UDP packet to the LDP port at the
> group multicast address for all routers on the subnet. When ldp-tunneling
> is turned on LDP treats the RSVP signaled LSPs as single hops. This allows
> an LDP speaker to establish a session with the router at the far end of
> the RSVP signaled LSP.
> The Junos manual says that LDP control packets are routed hop-by-hop
> rather than carried through the LSP. How does a UDP packet destined for a
> Multicast address get routed to the LDP speaker at the end of the RSVP
> signaled LSP if it's not being sent over the RSVP signaled LSP ?
>
>
> Snippet from the interfaces, class of service and firewall manual (5.0):
> When you configure the router to run LDP across RSVP-established LSPs, LDP
> will automatically establish sessions with the router at the other end of
> the LSP. LDP control packets are routed hop-by-hop, rather than carried
> through the LSP. This allows you to use simplex (one-way)
> traffic-engineered LSPs. Traffic in the opposite direction flows through
> LDP-established LSPs that follow unicast routing rather than through
> traffic-engineered tunnels.
>
>
> Tyler J. Wessels
> Tier III IP Engineer
> Level(3) Communications
>
>
This archive was generated by hypermail 2b29 : Mon Aug 05 2002 - 10:42:38 EDT