Question 1mediummultiple choice
Full question →A network engineer must summarize the following routes before advertising them upstream:
172.16.32.0/24 172.16.33.0/24 172.16.34.0/24 172.16.35.0/24
Which summary route should be used?
mediummultiple choiceObjective-mapped
A network engineer must summarize the following routes before advertising them upstream:
172.16.32.0/24 172.16.33.0/24 172.16.34.0/24 172.16.35.0/24
Which summary route should be used?
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
Best answer
172.16.32.0/22
Correct. This is correct. A /22 beginning at 172.16.32.0 cleanly includes 172.16.32.0 through 172.16.35.255, which matches the four listed /24 networks exactly.
B
Distractor review
172.16.32.0/23
A /23 covers only two /24 networks, not four. It would include the first half of the list but would leave out 172.16.34.0/24 and 172.16.35.0/24. This is a common mistake when candidates focus on the first two entries and forget to account for the whole range.
C
Distractor review
172.16.32.0/21
A /21 is larger than needed. Although it would include the required routes, it would also include additional address space beyond them. Overly broad summaries can create inaccurate advertisements and make troubleshooting harder, so exam questions usually expect the smallest correct summary that cleanly fits the listed routes.
D
Distractor review
172.16.34.0/22
The prefix length looks appealing because /22 is the right size, but the network boundary is wrong. Summary routes must start at the proper boundary for the mask length. A /22 block must begin on a multiple-of-four boundary in the third octet, which makes 172.16.32.0 valid and 172.16.34.0 invalid.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A frequent mistake is selecting a summary route that does not start on the correct subnet boundary for the mask length. For example, 172.16.34.0/22 looks like a valid /22 summary but is invalid because the third octet (34) is not divisible by 4. This misalignment causes the summary to include incorrect address ranges or exclude intended networks. Another trap is choosing a /23 mask, which only covers two /24 networks, missing half the routes. Candidates often focus on the first two networks and forget to verify the entire range, leading to incomplete summarization.
Technical deep dive
How to think about this question
Route summarization is a technique used in IP routing to combine multiple contiguous networks into a single advertisement, reducing the size of routing tables and improving routing efficiency. In IPv4, summarization works by finding a common prefix that covers all the individual networks. The prefix length (subnet mask) determines how many addresses are included in the summary. For example, a /22 prefix covers 1024 addresses, or four contiguous /24 networks. To create a valid summary route, the starting address must align with the subnet mask boundary. For a /22 mask, the third octet must be a multiple of 4 because each /22 block covers four /24 networks. In this question, the four /24 networks 172.16.32.0/24 through 172.16.35.0/24 fit perfectly into the 172.16.32.0/22 summary. Choosing a smaller mask like /23 would exclude some networks, while a larger mask like /21 would include unnecessary addresses. A common exam trap is misaligning the summary route with the subnet boundary, such as using 172.16.34.0/22, which is invalid because 34 is not a multiple of 4. This misalignment causes incorrect routing advertisements and can lead to routing loops or blackholes. Practically, Cisco routers require proper alignment for summarization to work correctly, making the correct boundary critical for both exam and real-world network design.
KKey Concepts to Remember
- Route summarization combines multiple contiguous networks into a single advertisement to reduce routing table size and improve efficiency.
- A summary route must start at a subnet boundary aligned with its mask length, ensuring the address is a multiple of the block size.
- A /22 subnet mask covers exactly four consecutive /24 networks, making it ideal for summarizing four contiguous /24 routes.
- Choosing a summary mask that is too small excludes some networks, while a mask that is too large includes unnecessary addresses.
- Cisco routers require proper subnet alignment for summarization to function correctly and avoid routing errors.
- Incorrect summary boundaries can cause routing loops, blackholes, or incomplete route advertisements in a network.
- The third octet in a /22 summary must be divisible by 4 to align with the subnet boundary and cover four /24 networks.
- Summarization improves routing protocol scalability by minimizing the number of routes advertised upstream.
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?
Route summarization combines multiple contiguous networks into a single advertisement to reduce routing table size and improve efficiency.
What is the correct answer to this question?
The correct answer is: 172.16.32.0/22 — The correct summary is 172.16.32.0/22 because a /22 covers exactly four consecutive /24 networks when the starting boundary is aligned correctly. This is the part many people miss: summarization is not only about how many networks fit into a block, but also where that block starts. Here the four /24 networks begin neatly at 172.16.32.0 and continue through 172.16.35.255, which is the exact range a /22 covers. A /23 would be too small, while a /21 would be unnecessarily broad and could advertise addresses you do not intend to include. The /22 beginning at 172.16.34.0 is not on a valid /22 boundary, so that option is misaligned.
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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