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MPLS Fundamentals by Luc De Ghein - CCIE No. 1897

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Label Encoding

ATM switches that are running MPLS are still switching ATM cells. As such, they cannot forward labeled frames. Because the MPLS labels are mapped to VCs in the ATM cloud, the MPLS label value is mapped to the VPI/VCI pair. If the labeled packet has a label stack with more than one label, only the value of the top label is mapped to the VPI/VCI fields. Figure 5-4 shows the MPLS label mapped to the VPI/VCI values.

Figure 5-4. Label Encoding

When the edge ATM LSR receives a frame, the frame is chopped up into cells. Only the top label value is encoded as a VPI/VCI value. The rest of the labels in the label ...

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