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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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Fragmentation of MPLS Packets

If an LSR receives a labeled packet that is too big to be sent out on a data link, the packet should be fragmented. This is similar to fragmenting an IP packet. If a labeled packet is received and the LSR notices that the outgoing MTU is not big enough for this packet, the LSR strips off the label stack, fragments the IP packet, puts the label stack (after the pop, swap, or push operation) onto all fragments, and forwards the fragments. Only if the IP header has the Don't Fragment (DF) bit set does the LSR not fragment the IP packet, but it drops the packet and returns an ICMP error message "Fragmentation needed and do not fragment bit set" (ICMP type 3, code 4) to the originator of the IP packet. As with the ICMP message "time exceeded" (type 11, code 0), which is sent when the TTL expires of a labeled packet, the "Fragmentation needed and do not fragment bit set" ICMP message is sent, using a label stack that is the outgoing label stack for the packet that caused the ICMP message to be created. This means that the ICMP message travels further down the LSP until it reaches the egress LSR of that LSP. Then it is returned to the originator of the packet with the DF bit set.

In general, fragmentation causes a performance impact and should be avoided. A good method to avoid fragmentation is using the Path MTU Discovery method as described in the next section.

7. Path MTU Discovery | Next Section Previous Section