MPLS Label Spaces

In Figure 2-10, LSR A can advertise label L1 for FEC 1 to LSR B and label L1 for FEC 2 to LSR C, but only if LSR A can later distinguish from which LSR the packet with label L1 was received. In the case that LSR B and LSR C are directly connected to LSR A via point-to-point links, this can easily be achieved by the MPLS implementation on the LSR. The fact that the label L1 is only unique per interface lends its name to this label scope: per-interface label space. If per-interface label space is used, the packet is not forwarded solely based on the label, but based on both the incoming interface and the label.

Figure 2-10 Per-Interface Label Space

FEC 1

FEC 2

LSR C

The other possibility is that the label is not unique per interface, but over the LSR assigning the label. This is called per-platform label space. In that case, LSR A distributes FEC 1 with label L1 to LSRs B and C, as you can see in Figure 2-11. When LSR A distributes a label for FEC 2, this label must be a different label than label L1. If per-platform label space is used, the packet is forwarded solely based on the label, independently from the incoming interface.

Figure 2-11 Per-Platform Label Space

LSR C

LSR C

FEC 1

In Cisco IOS, all Label Switching Controlled-ATM (LC-ATM) interfaces have a per-interface label space, whereas all ATM frame-based and non-ATM interfaces have a per-platform label space. Refer to Chapter 5 for more details on LC-ATM interfaces.

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