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MPLS Fundamentals by Luc De Ghein - CCIE No. 1897

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MPLS Payload

The MPLS label has no Network Level Protocol identifier field. This field is present in all Layer 2 frames to indicate what the Layer 3 protocol is. How does the LSR know what the protocol is behind the label stack? Or, in other words, how does the LSR know what the MPLS payload is? Most LSRs do not need to know, because they will receive a labeled packet, swap the top label, and send the packet on the outgoing link. This is the case for intermediate LSRs or P routers.

Intermediate LSRs do not need to know what the MPLS payload is because all the information needed to switch the packet is known by looking at the top label only. If the label stack consists of more than one label, the labels below the top label might not be assigned ...

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