4.1.a [ii] LDP
MPLS LDP provides the means for LSRs to request, distribute, and release label prefix binding information to peer routers in a network. LDP enables LSRs to discover potential peers and to establish LDP sessions with those peers for the purpose of exchanging label binding information.
MPLS LDP enables one LSR to inform another LSR of the label bindings it has made. Once a pair of routers communicate the LDP parameters, they establish a label-switched path (LSP). MPLS LDP enables LSRs to distribute labels along normally routed paths to support MPLS forwarding. This method of label distribution is also called hop-by-hop forwarding. With IP forwarding, when a packet arrives at a router the router looks at the destination address in the IP header, performs a route lookup, and forwards the packet to the next hop. With MPLS forwarding, when a packet arrives at a router the router looks at the incoming label, looks up the label in a table , and then forwards the packet to the next hop. MPLS LDP is useful for applications that require hop-by-hop forwarding, such as MPLS VPNs.
When you enable MPLS LDP, the LSRs send out messages to try to find other LSRs with which they can create LDP sessions. An LSR engages in discovery by periodically transmitting LDP Hello messages to signal its desire to advertise label bindings. The LSR sends the LDP Hello messages as UDP packets to the well known LDP port (646).
LDP defines two types of discovery:
● Basic discovery—Used to discover directly connected LDP LSRs. For basic discovery, an LSR sends Hellos messages to the “all routers on this subnet” multicast address on interfaces for which LDP has been configured.
● Extended discovery—Used between nondirectly connected LDP LSRs. For extended discovery, an LSR sends targeted Hello messages to a specific IP address.
The Hello messages carry the LDP ID of the label space that the sending LSR wants to advertise , as well as other information. When an LSR receives an LDP Hello message from another LSR, it considers that LSR and the specified label space to be “discovered.” After two LSRs discover each other in this manner, they attempt to establish an LDP session
Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 4373-4384). . Kindle Edition.