IPv6 removed fragmentation in routers. Explain how IPv6 deals with different maximum transmission units (MTUs) of different links along a path. What is the advantage of this approach when compared to IPv4 fragmentation?
Try an answer before revealing the guidance below.
Key Concepts
- Path MTU discovery
- ICMPv6 Packet Too Big
- Source fragmentation
- Router work
Answer Approach
- Ask what an IPv6 router does with a packet larger than the next link’s MTU.
- Explain how the sender learns the usable size.
- Compare the per-packet work routers do in IPv4 versus IPv6.
Full Answer
Answer status: Verified against study notes. Revision notes, not an official marking scheme.
An IPv6 router that cannot forward an oversized packet drops it and sends an ICMPv6 Packet Too Big message with the supported MTU. The source then sends smaller packets, or uses an IPv6 Fragment header if fragmentation is needed. Routers no longer fragment each oversized packet, reducing router work and speeding up forwarding, with reassembly left to the destination.
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