Nondirectly Connected MPLS LDP Sessions

If the LSR is more than one hop from its neighbor, it is nondirectly connected to its neighbor. For these
nondirectly connected neighbors, the LSR sends out a targeted Hello message as a UDP packet, but as a
unicast message specifically addressed to that LSR. The nondirectly connected LSR responds to the Hello
message and the two routers begin to establish an LDP session. This is called extended discovery.
An MPLS LDP targeted session is a label distribution session between routers that are not directly
connected. When you create an MPLS traffic engineering tunnel interface, you need to establish a label
distribution session between the tunnel headend and the tailend routers. You establish nondirectly
connected MPLS LDP sessions by enabling the transmission of targeted Hello messages.
You can use the mpls ldp neighbor targetedcommand to set up a targeted session when other means of
establishing targeted sessions do not apply, such as configuring mpls ipon a traffic engineering (TE) tunnel
or configuring Any Transport over MPLS (AToM) virtual circuits (VCs). For example, you can use this
command to create a targeted session between directly connected MPLS label switch routers (LSRs) when
MPLS label forwarding convergence time is an issue.
The mpls ldp neighbor targetedcommand can improve label convergence time for directly connected
neighbor LSRs when the link(s) directly connecting them are down. When the links between the neighbor
LSRs are up, both the link and targeted Hellos maintain the LDP session. If the links between the neighbor
LSRs go down, the targeted Hellos maintain the session, allowing the LSRs to retain labels learned from
each other. When a link directly connecting the LSRs comes back up, the LSRs can immediately reinstall
labels for forwarding use without having to reestablish their LDP session and exchange labels.
The exchange of targeted Hello messages between two nondirectly connected neighbors can occur in
several ways, including the following:
• Router 1 sends targeted Hello messages carrying a response request to Router 2. Router 2 sends
targeted Hello messages in response if its configuration permits. In this situation, Router 1 is
considered to be active and Router 2 is considered to be passive.
• Router 1 and Router 2 both send targeted Hello messages to each other. Both routers are considered to
be active. Both, one, or neither router can also be passive, if they have been configured to respond to
requests for targeted Hello messages from each other.
The default behavior of an LSR is to ignore requests from other LSRs that send targeted Hello messages.
You can configure an LSR to respond to requests for targeted Hello messages by issuing the mpls ldp
discovery targeted-hello accept command.
The active LSR mandates the protocol that is used for a targeted session. The passive LSR uses the protocol
of the received targeted Hello messages.
For information about creating MPLS LDP targeted sessions, see the Establishing Nondirectly Connected
MPLS LDP Sessions, page 8.
Introduction to LDP Label Bindings Label Spaces and LDP Identifiers
An LDP label binding is an association between a destination prefix and a label. The label used in a label
binding is allocated from a set of possible labels called a label space.
LDP supports two types of label spaces:
• Interface-specific--An interface-specific label space uses interface resources for labels. For example,
label-controlled ATM (LC-ATM) interfaces use virtual path identifiers/virtual circuit identifiers (VPIs/
VCIs) for labels. Depending on its configuration, an LDP platform may support zero, one, or more
interface-specific label spaces.
Introduction to LDP Label Bindings Label Spaces and LDP Identifiers
Nondirectly Connected MPLS LDP Sessions
MPLS LDP Configuration Guide, Cisco IOS Release 12.4
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