MPLS Fundamentals

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 ...