Question 1mediummultiple choice
Full question →mediummultiple choiceObjective-mapped
Which field is modified by each router hop in an IPv4 packet to prevent endless forwarding loops?
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
Source port
Routers do not decrement transport-layer source ports.
B
Best answer
TTL
Correct. TTL is reduced each time the packet crosses a router.
C
Distractor review
Sequence number
Sequence numbers are transport-layer values, not hop-limit controls.
D
Distractor review
CRC in the Ethernet trailer
The frame CRC is recalculated on each segment, but it is not the IP loop-prevention field.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A frequent exam trap is selecting transport-layer fields like source port or sequence number as the field modified by each router hop. These fields are part of TCP or UDP headers and remain unchanged by routers during forwarding. Another common mistake is confusing the Ethernet frame CRC with the TTL; while the CRC is recalculated on each link to verify frame integrity, it does not control packet lifetime or prevent routing loops. Misunderstanding these distinctions can lead to incorrect answers about how routers manage packet forwarding and loop prevention.
Technical deep dive
How to think about this question
The Time to Live (TTL) field is a fundamental component of the IPv4 packet header designed to limit the lifespan of a packet in a network. It is an 8-bit field that specifies the maximum number of router hops a packet can traverse before being discarded. Each router that forwards the packet decrements the TTL value by one. This mechanism prevents packets from circulating indefinitely in the event of routing loops, which can cause network congestion and degrade performance. When a router receives an IPv4 packet, it examines the TTL field. If the TTL is greater than one, the router decrements the TTL by one and forwards the packet to the next hop. If the TTL reaches zero, the router discards the packet and typically generates an ICMP Time Exceeded message to inform the sender that the packet was dropped due to TTL expiration. This behavior is consistent across Cisco routers and is a critical part of IP routing and forwarding logic. A common exam trap is confusing the TTL field with transport-layer fields such as source port or sequence number, which do not influence packet forwarding or loop prevention. Additionally, some may mistakenly think the Ethernet frame CRC affects packet lifetime; however, the CRC is recalculated on each link and only ensures frame integrity on that segment. Understanding the TTL’s role helps network engineers troubleshoot routing issues and design networks that avoid persistent loops and packet storms.
KKey Concepts to Remember
- The Time to Live (TTL) field in an IPv4 packet header decrements by one at each router hop to prevent infinite packet forwarding loops.
- Routers use the TTL field to determine when to discard a packet that has traversed too many hops, ensuring network stability and loop prevention.
- The TTL field is an 8-bit value that starts with a set number and decreases by one with each router forwarding action.
- If the TTL value reaches zero, the router discards the packet and typically sends an ICMP Time Exceeded message back to the sender.
- The TTL mechanism operates independently of transport-layer fields such as source port or sequence number, which do not affect routing decisions.
- The TTL field is part of the IPv4 header and is critical for routing protocols and IP forwarding behavior in Cisco devices.
- Routers recalculate the Ethernet frame CRC on each link, but this does not influence IP packet forwarding or loop prevention.
- Understanding the TTL field’s role helps differentiate it from other packet fields that do not control packet lifetime or routing loops.
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.
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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 packet header decrements by one at each router hop to prevent infinite packet forwarding loops.
What is the correct answer to this question?
The correct answer is: TTL — The Time to Live field is decremented at each hop.
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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