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MPLS Fundamentals by Luc De Ghein - CCIE No. 1897

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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

The other possibility is that the label is not unique per interface, but ...

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