Terminology
LDP session
Two LSRs establish a TCP-based LDP session to exchange FEC-label mappings.
LDP peer
Two LSRs that use LDP to exchange FEC-label mappings are LSR peers.
Label spaces and LDP identifiers
Label spaces include the following types:
Per-interface label space—Each interface uses a single, independent label space. Different interfaces can use the same label values.
Per-platform label space—Each LSR uses a single label space. The device only supports the per-platform label space.
A six-byte LDP Identifier (LDP ID) identifies a label space on an LSR. It is in the format of <LSR ID>:<label space number>, where:
The LSR ID takes four bytes to identity the LSR.
The label space number takes two bytes to identify a label space within the LSR.
A label space number of 0 indicates that the label space is a per-platform label space. A label space number other than 0 indicates a per-interface label space.
FECs and FEC-label mappings
MPLS groups packets with the same characteristics (such as the same destination or service class) into a class, called an FEC. The packets of the same FEC are handled in the same way on an MPLS network.
LDP can classify FECs by destination IP address.
An LSR assigns a label for an FEC and advertises the FEC-label mapping, or FEC-label binding, to its peers in a Label Mapping message.