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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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Path MTU Discovery

One method to avoid fragmentation is Path MTU Discovery, which most modern IP hosts perform automatically. In that case, the IP packets sent out have the "Don't Fragment" (DF) bit set. When a packet encounters a router that cannot forward the packet without fragmenting it, the router notices that the DF bit is set, drops the packet, and sends an ICMP error message "Fragmentation needed and do not fragment bit set" (ICMP type 3, code 4) to the originator of the IP packet. The originator of the IP packet then lowers the size of the packet and retransmits the packet. If a problem still exists, the host can lower the size of the packet again. This continues until no ICMP message is received for the IP packet. The size of the last IP packet successfully sent is then used as maximum packet size for all subsequent IP traffic between the specific source and destination; hence, it is the MTU of the path.