Label Distribution with LDP

For every IGP IP prefix in its IP routing table, each LSR creates a local binding—that is, it binds a label to the IPv4 prefix. The LSR then distributes this binding to all its LDP neighbors. These received bindings become remote bindings. The neighbors then store these remote and local bindings in a special table, the label information base (LIB). Each LSR has only one local binding per prefix, at least when the label space is per platform. If the label space is per interface, one local label binding can exist per prefix per interface. Therefore, you can have one label per prefix or one label per prefix per interface, but the LSR gets more than one remote binding because it usually has more than one adjacent LSR.

NOTE The difference between per-platform and per-interface label space is explained in the later section "MPLS Label Spaces."

Out of all the remote bindings for one prefix, the LSR needs to pick only one and use that one to determine the outgoing label for that IP prefix. The routing table (sometimes called the routing instance base, or RIB) determines what the next hop of the IPv4 prefix is. The LSR chooses the remote binding received from the downstream LSR, which is the next hop in the routing table for that prefix. It uses this information to set up its label forwarding information base (LFIB) where the label from the local binding serves as the incoming label and the label from the one remote binding chosen via the routing table serves as the outgoing label. Therefore, when an LSR receives a labeled packet, it is now capable of swapping the incoming label it assigned, with the outgoing label assigned by the adjacent next-hop LSR. Figure 2-8 shows the advertisement by LDP of the bindings between the LSRs for the IPv4 prefix 10.0.0.0/8. Each LSR allocates one label per IPv4 prefix. The local binding is this one prefix and its associated label.

Label Forwarding Instance Base 35

Figure 2-8 An IPv4-over-MPLS Network Running LDP

Remote Binding From Downstream Neighbor

Remote Binding From Downstream Neighbor

Remote Binding From Downstream Neighbor

10.0.0.0/8 Label 129

IGP Prefix 10.0.0.0/8

10.0.0.0/8 Label 129

Figure 2-9 shows the IPv4 packet—destined for 10.0.0.0/8—entering the MPLS network on the ingress LSR, where it is imposed with the label 129 and switched toward the next LSR. The second LSR swaps the incoming label 129 with the outgoing label 17 and forwards the packet toward the third LSR. The third LSR swaps the incoming label 17 with the outgoing label 33 and forwards the packet to the next LSR and so on.

Figure 2-9 shows the IPv4 packet—destined for 10.0.0.0/8—entering the MPLS network on the ingress LSR, where it is imposed with the label 129 and switched toward the next LSR. The second LSR swaps the incoming label 129 with the outgoing label 17 and forwards the packet toward the third LSR. The third LSR swaps the incoming label 17 with the outgoing label 33 and forwards the packet to the next LSR and so on.

Figure 2-9 An IPv4-over-MPLS Network Running LDP: Packet Switching

IPv4 Packet

Label 129

Label 17

IPv4 Packet

IPv4 Packet

Label 33

IPv4 Packet

Ingress LSR

NOTE In Cisco IOS, LDP does not bind labels to BGP IPv4 prefixes.

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