Exchanging Label Bindings - Cisco CRS Configuration Manual

Overview of Label Distribution Protocol
LDP uses the hello discovery mechanism to discover its neighbor or peer on the network. When LDP is
enabled on an interface, it sends hello messages to a link-local multicast address, and joins a specific multicast
group to receive hellos from other LSRs present on the given link. When LSRs on a given link receive hellos,
their neighbors are discovered and the LDP session (using TCP) is established.
Note Hellos are not only used to discover and trigger LDP sessions; they are also required to maintain LDP
sessions. If a certain number of hellos from a given peer are missed in sequence, LDP sessions are brought
down until the peer is discovered again.
LDP also supports non-link neighbors that could be multiple hops away on the network, using the targeted
hello mechanism. In these cases, hellos are sent on a directed, unicast address.
The first message in the session establishment phase is the initialization message, which is used to negotiate
session parameters. After session establishment, LDP sends a list of all its interface addresses to its peers in
an address message. Whenever a new address becomes available or unavailable, the peers are notified regarding
such changes via ADDRESS or ADDRESS_WITHDRAW messages respectively.
When MPLS LDP learns an IGP prefix it allocates a label locally as the inbound label. The local binding
between the prefix label is conveyed to its peers via LABEL_MAPPING message. If the binding breaks and
becomes unavailable, a LABEL_WITHDRAW message is sent to all its peers, which responds with
LABEL_RELEASE messages.
The local label binding and remote label binding received from its peer(s) is used to setup forwarding entries.
Using routing information from the IGP protocol and the forwarding information base (FIB), the next active
hop is selected. Label binding is learned from the next hop peer, and is used as the outbound label while setting
up the forwarding plane.
The LDP session is also kept alive using the LDP keepalive mechanism, where an LSR sends a keepalive
message periodically to its peers. If no messages are received and a certain number of keepalive messages
are missed from a peer, the session is declared dead, and brought down immediately.
Related Topics
Configuring LDP Discovery Parameters, on page 25
Configuring LDP Discovery Over a Link, on page 27
Configuring LDP Link: Example, on page 80
Configuring LDP Discovery for Active Targeted Hellos, on page 29
Configuring LDP Discovery for Passive Targeted Hellos, on page 32
Configuring LDP Discovery for Targeted Hellos: Example, on page 80

Exchanging Label Bindings

LDP creates LSPs to perform the hop-by-hop path setup so that MPLS packets can be transferred between
the nodes on the MPLS network.
Cisco IOS XR MPLS Configuration Guide for the Cisco CRS Router, Release 5.1.x
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Implementing MPLS Label Distribution Protocol