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350-401 · Question #755
350-401 Question #755: Real Exam Question with Answer & Explanation
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.
Submitted by thandi_sa· Mar 6, 2026IP Services
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
Explanation
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.
Common mistakes.
- A. Time Exceeded messages (ICMPv6 Type 3) are used for hop limit expiry or fragment reassembly timeout, not for MTU issues.
- C. 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.
- D. Multicast Termination Router is not a standard ICMP message type used in IPv6 Path MTU Discovery.
Concept tested. IPv6 Path MTU Discovery ICMPv6
Topics
#IPv6 PMTUD#ICMPv6#MTU discovery#Packet Too Big
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