to all other connected LSRs as soon as a new prefix is added to the IP routing table of the LSR. For an example, see Figure 13.9 where a new prefix, 10.11.15.0/24, has been advertised by an Inte- rior Gateway Protocol (IGP). As soon as an LSR has this new prefix in its routing table, it sends a label-mapping message to its upstream neighbor. Bind 10.11. 15.0/24 Bind 11.15.0/24 to 63 to 42 Bind 10.11.15.0/24 Bind 10.11.15.0/24 to 35 to 75 Bind 10.11.15.0/24 to 54 Bind 10.11.15.0/24 to 91 Traffic direction Label-mapping message direction Fig. 13.9 LDP mapping messages for a new FEC (IP address prefix 10.11.15.0/24). Traffic Engineering The label-mapping message contains a label value and an encoded form of the FEC that this label applies to. For example, C sends to B the label mapping to bind the FEC defined by the IP address prefix 10.11.15.0/24 to the label 42. Then, if B receives unlabeled packets belonging to this FEC, it should label them with 42 before sending them to C. This label will be placed, in the appropriate NHLFE in B. Of course, B will also send a label- mapping message to A; in the example B binds label 63 to this prefix and will put 63 into its ILM and map this to the NHLFE of 42. This process continues until a complete LSP is created. In the mode of operation described here, LSRs will independently send label- mapping messages as soon as the prefix appears in the IP routing table and send the message to all immediate neighbors. An LSR that receives the label mapping will record this in its NHLFE even if it does not know about the prefix yet (as it might at some future time). In Figure 13.9 the mapping messages are shown only in one direction to emphasize that the mappings are generated by the downstream LSR. However, there is no notion of split horizon in LDP, SO B also sends a label-mapping message for this FEC to C, even though C does not have immediate use for it, and C keeps this mapping. This is useful if there is a change in topology because label bindings will preexist. When LDP operates a