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101 · Question #643
101 Question #643: Real Exam Question with Answer & Explanation
The correct answer is D: The TTL field of the IP header was decremented to 0. and this was consequently dropped by a. ICMP Type 11 (Time Exceeded) is generated when a router decrements the TTL field of an IP packet to 0 and must discard it. This mechanism prevents packets from looping indefinitely on a network.
Question
An administrator is designing a remote host connection using an ICMP based network troubleshooting utility and receive an ICMP error diagram with the following error code 11. Times exceeded. What does this mean?
Options
- AA faulty router on the network is unable to process the packet in a timely manner
- BA switch on the network the frame due to an incorrect checksum
- CThe TTL field of the IP header was set to 255. and this was consequently dropped by a router
- DThe TTL field of the IP header was decremented to 0. and this was consequently dropped by a
Explanation
ICMP Type 11 (Time Exceeded) is generated when a router decrements the TTL field of an IP packet to 0 and must discard it. This mechanism prevents packets from looping indefinitely on a network.
Common mistakes.
- A. ICMP Time Exceeded is not related to router processing speed or delays - it is triggered strictly by the TTL counter reaching zero, not by a performance failure.
- B. Incorrect checksums at Layer 2 cause a switch to silently drop the frame without generating any ICMP error message back to the sender.
- C. A TTL value of 255 is simply the maximum initial value and does not cause the packet to be dropped; only reaching 0 triggers the Time Exceeded error.
Concept tested. ICMP Type 11 Time Exceeded and TTL behavior
Reference. https://www.rfc-editor.org/rfc/rfc792
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