350-401 · Question #755
In IPv6 Path MTU Discovery, which ICMP message is sent by an intermediary router that requires a smaller MTU?
The correct answer is B. Packet Too Big. In IPv6 Path MTU Discovery, an intermediary router that needs a smaller MTU discards the packet and sends an ICMPv6 Packet Too Big message to the source.
Question
In IPv6 Path MTU Discovery, which ICMP message is sent by an intermediary router that requires a smaller MTU?
Options
- ATime Exceeded, with code 1 (fragment reassembly time exceeded)
- BPacket Too Big
- CDestination Unreachable, with code 4 (the datagram is too big)
- DMulticast Termination Router
How the community answered
(63 responses)- A3% (2)
- B89% (56)
- C2% (1)
- D6% (4)
Why each option
In IPv6 Path MTU Discovery, an intermediary router that needs a smaller MTU discards the packet and sends an ICMPv6 Packet Too Big message to the source.
Time Exceeded messages (ICMPv6 Type 3) are used for hop limit expiry or fragment reassembly timeout, not for MTU issues.
IPv6 routers do not fragment packets; instead, if an IPv6 packet exceeds the MTU of the next-hop link, the router discards the packet and sends an ICMPv6 Type 2 'Packet Too Big' message back to the source, including the link's MTU.
The ICMPv4 message 'Destination Unreachable, code 4 (fragmentation needed and DF set)' is the IPv4 equivalent for MTU issues, but IPv6 uses the specific 'Packet Too Big' message.
Multicast Termination Router is not a standard ICMP message type used in IPv6 Path MTU Discovery.
Concept tested: IPv6 Path MTU Discovery ICMPv6
Source: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipv6/configuration/15-mt/ipv6-15-mt-book/ip6-path-mtu-disc.html
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