Question 1easymultiple choice
Full question →easymultiple choiceObjective-mapped
Which field in an IPv4 packet is primarily used to prevent packets from looping forever in the network?
Answer choices
Why each option matters
Good practice is not just finding the correct option. The wrong answers often show the exact trap the exam wants you to fall into.
A
Distractor review
Version
Version identifies IPv4 versus IPv6.
B
Distractor review
Header checksum
The checksum detects header corruption.
C
Best answer
Time to Live
TTL is the loop-prevention field.
D
Distractor review
Protocol
Protocol identifies the payload such as TCP or UDP.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A common exam trap is mistaking the Protocol or Header Checksum fields for the loop prevention mechanism. The Protocol field only identifies the payload type, such as TCP or UDP, and does not affect packet forwarding or lifespan. The Header Checksum ensures header integrity but does not prevent loops. Another trap is confusing the Version field, which merely distinguishes IPv4 from IPv6. Candidates might incorrectly select these fields because they are prominent in the IPv4 header, but only TTL controls how long a packet can remain in the network before being discarded. Understanding this distinction is critical to avoid selecting incorrect answers on the CCNA exam.
Technical deep dive
How to think about this question
The Time to Live (TTL) field in an IPv4 packet header is a crucial mechanism designed to prevent packets from circulating indefinitely in a network. TTL is an 8-bit field that specifies the maximum number of hops (routers) a packet can traverse before being discarded. Each router that forwards the packet decrements the TTL value by one. When the TTL reaches zero, the packet is dropped, effectively stopping routing loops and conserving network resources. This TTL decrement rule ensures that packets caught in routing loops do not consume bandwidth endlessly. The TTL field is fundamental in routing protocols and network troubleshooting tools like traceroute, which relies on TTL expiration to map network paths. Cisco devices strictly adhere to this TTL decrement process, making it a reliable loop prevention method in IPv4 networks. A common exam trap is confusing TTL with other IPv4 header fields such as the Protocol or Header Checksum fields. While Protocol identifies the encapsulated transport layer protocol and Header Checksum verifies header integrity, neither controls packet lifespan. Understanding TTL’s unique role in loop prevention and its behavior in Cisco routers is essential for correctly answering related CCNA questions and for practical network design and troubleshooting.
KKey Concepts to Remember
- The Time to Live (TTL) field in an IPv4 header prevents packets from looping indefinitely by limiting the number of hops a packet can traverse.
- Each router decrements the TTL value by one before forwarding the packet, ensuring eventual packet discard if loops occur.
- When the TTL value reaches zero, the router discards the packet and typically sends an ICMP Time Exceeded message back to the sender.
- The Version field identifies whether the packet is IPv4 or IPv6 and does not affect packet lifespan or routing loops.
- The Header Checksum field verifies the integrity of the IPv4 header but does not influence packet forwarding or loop prevention.
- The Protocol field specifies the encapsulated transport protocol (e.g., TCP, UDP) and is unrelated to loop prevention.
- Cisco routers implement TTL decrementing consistently, making TTL a reliable mechanism for loop prevention in IPv4 networks.
- Routing loops can cause network congestion and outages, which TTL helps mitigate by ensuring packets do not circulate endlessly.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Related practice questions
Related 200-301 practice-question pages
Use these pages to review the topic behind this question. This is how one missed question becomes focused revision.
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More questions from this exam
Keep practising from the same exam bank, or move into a focused topic page if this question exposed a weak area.
Question 1
A router learns the same prefix from both OSPF and EIGRP. Which route is installed by default?
Question 2
A router shows this output: R1#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 10.1.1.2 1 FULL/DR 00:00:34 192.168.12.2 GigabitEthernet0/0 10.1.1.3 1 2WAY/DROTHER 00:00:39 192.168.12.3 GigabitEthernet0/0 Which statement is correct?
Question 3
What is the OSPF metric called?
Question 4
A non-root switch has two uplinks toward the root bridge. One path has a lower total STP cost than the other. What role will the lower-cost uplink have?
Question 5
A router interface applies this ACL inbound: 10 deny tcp any any eq 80 20 permit ip any any A user reports that web browsing to a server by IP address fails, but ping works. Which statement best explains the behavior?
Question 6
A router learns route 198.51.100.0/24 from OSPF with AD 110 and also has a static route to the same prefix configured with AD 150. Which route is installed?
FAQ
Questions learners often ask
What does this 200-301 question test?
The Time to Live (TTL) field in an IPv4 header prevents packets from looping indefinitely by limiting the number of hops a packet can traverse.
What is the correct answer to this question?
The correct answer is: Time to Live — Each router decrements the TTL field by one. When TTL reaches zero, the packet is discarded. That mechanism prevents indefinite looping.
What should I do if I get this 200-301 question wrong?
Then try more questions from the same exam bank and focus on understanding why the wrong options are tempting.
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