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MPLS Fundamentals: Forwarding Labeled Packets

Chapter Description

Forwarding labeled packets is quite different from forwarding IP packets in that not only is the IP lookup replaced with a lookup of the label in the label forwarding information base (LFIB), but different label operations are also possible. This chapter specifically focuses on how labeled packets are forwarded in an MPLS network.

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MPLS Fundamentals

MPLS Fundamentals

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In this chapter, you have learned how a packet is forwarded in an MPLS network. You have seen that CEF can label an incoming IP packet. An incoming labeled packet is forwarded by looking up the top label value in the LFIB and finding the label operation and next hop to forward the packet to. You have learned that several specially reserved labels (implicit NULL label, explicit NULL label, and Router Alert label) exist and what their function is. MPLS has its own MTU, which is important because the labeled packet is slightly bigger than the unlabeled one. That is because only a few labels add to the packet. You have seen that MPLS has a TTL field in the labels that is used in the same way as it is used for IP packets, but there is a specific behavior in copying the IP TTL to the label TTL fields and vice versa. Finally, you have seen that MPLS also supports fragmenting labeled packets.

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