Label Distribution and Label Tables
Routers send label information to each other using LDP, but they must first have
a route to a network before creating a label for it. The process has four steps:
Step 1. Routing protocols distribute route information. The routing table
and CEF FIB are built based on this information.
Step 2. The LSR assigns a locally significant label to each destination
network. This is recorded in the LIB table. Label values 0
through 15 are reserved for special use.
Step 3. Each LSR sends the local label for each network to its neighbors
via LDP. This is done asynchronously.a router does not wait to
receive a label from its downstream router before advertising its
own label. Labels are advertised to every neighbor, even the one
chosen as the next hop for that network.
Step 4. The LSR records label information received from its neighbors in
its LIB, FIB, and LFIB.
Each forwarding table is used as follows:
�¡ The LIB lists each IP network, the local label for that network, and
any labels received from neighbors for that network. It is in the
control plane.
�¡ The FIB is used to forward unlabeled IP packets. It is in the data plane.
�¡ The LFIB lists each label, what label to swap it for, and the next-hop
neighbor. It is in the data plane.