Question 1,184 of 2,152

DHCP (IPv4 and IPv6) →mediumMultiple ChoiceObjective-mapped

Quick Answer

The answer is that the DHCP server lacks a route to the client subnet. When a DHCP relay agent forwards a DISCOVER message, it sets the giaddr field to its own interface IP on the client-facing subnet, and the server must unicast the OFFER back to that giaddr. Even though the server can ping the relay’s management IP, the return traffic to the giaddr subnet may fail if the server has no route for that specific network, causing the OFFER to be dropped. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of DHCP relay operation and the critical distinction between reachability of an interface IP versus a subnet route—a common trap where engineers assume a successful ping to the relay proves end-to-end connectivity. Remember: the server responds to the giaddr, not to the relay’s source IP, so a missing route to the client subnet silently breaks address assignment. A useful memory tip is “giaddr is the return address—route it or lose it.”

300-410 DHCP (IPv4 and IPv6) Practice Question

This 300-410 practice question tests your understanding of dhcp (ipv4 and ipv6). The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.

A network engineer is troubleshooting a DHCPv4 issue where a router configured as a DHCP server is not assigning addresses to clients on a subnet that is reachable via a different router (relay). The relay router (R2) has 'ip helper-address 10.1.1.1' on its client-facing interface, and the DHCP server is at 10.1.1.1 (R1). The engineer sees that R2 is sending DHCP DISCOVER messages with giaddr set to the client-facing interface IP, but R1 is not responding. R1 has a DHCP pool for the client subnet. The engineer pings 10.1.1.1 from R2 successfully. What is the most likely cause?

Clue words in this question

Noticing these words before you look at the options changes how you read each choice.

Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

Question 1mediummultiple choice

Read the full DHCP explanation →

A
The DHCP server does not have a route to the client subnet (the giaddr subnet).
Correct because the server sends the OFFER to the giaddr IP, which is on a different subnet; without a route back, the OFFER is lost.
B
The relay agent R2 is missing the 'ip dhcp relay information option' command.
Why wrong: Incorrect because Option 82 is not required for basic relay; its absence does not prevent the server from responding.
C
The DHCP pool on R1 is missing the 'default-router' command.
Why wrong: Incorrect because missing default-router would still allow the server to send an OFFER with an IP address; the client would just not have a gateway.
D
The 'ip helper-address' on R2 should point to the server's loopback address, not the interface IP.
Why wrong: Incorrect because the helper address can be any IP reachable on the server; the interface IP is fine.

Full breakdown with real-world context →

Answer choices

Why each option matters

Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.

Correct answer & explanation

✓

The DHCP server does not have a route to the client subnet (the giaddr subnet).

The DHCP server may not have a route back to the client subnet (the giaddr subnet). Even though the server's interface IP is reachable, the server needs to send the OFFER to the giaddr (which is the relay agent's interface IP). If the server does not have a route to that subnet, the OFFER will be dropped. The ping from R2 to R1 succeeds because R1's interface is directly connected, but the return traffic from R1 to the giaddr (which is on a different subnet) may fail if R1 does not have a route.

Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

✓
The DHCP server does not have a route to the client subnet (the giaddr subnet).
Why this is correct
Correct because the server sends the OFFER to the giaddr IP, which is on a different subnet; without a route back, the OFFER is lost.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
✗
The relay agent R2 is missing the 'ip dhcp relay information option' command.
Why it's wrong here
Incorrect because Option 82 is not required for basic relay; its absence does not prevent the server from responding.
✗
The DHCP pool on R1 is missing the 'default-router' command.
Why it's wrong here
Incorrect because missing default-router would still allow the server to send an OFFER with an IP address; the client would just not have a gateway.
✗
The 'ip helper-address' on R2 should point to the server's loopback address, not the interface IP.
Why it's wrong here
Incorrect because the helper address can be any IP reachable on the server; the interface IP is fine.

Common exam traps

Common exam trap: usable hosts are not the same as total addresses

Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.

Detailed technical explanation

How to think about this question

Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.

KKey Concepts to Remember

CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.

TExam Day Tips

Write the block size before choosing the subnet.
Check whether the question asks for hosts, subnets or a specific address range.
Do not confuse /24, /25, /26 and /27 host counts.

Key takeaway

Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.

Real-world example

How this comes up in practice

A network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.

What to study next

Got this wrong? Here's your next step.

Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 300-410 subnetting questions on CIDR, address ranges, and subnet selection.

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FAQ

Questions learners often ask

What does this 300-410 question test?

DHCP (IPv4 and IPv6) — This question tests DHCP (IPv4 and IPv6) — CIDR notation defines the prefix length..

What is the correct answer to this question?

The correct answer is: The DHCP server does not have a route to the client subnet (the giaddr subnet). — The DHCP server may not have a route back to the client subnet (the giaddr subnet). Even though the server's interface IP is reachable, the server needs to send the OFFER to the giaddr (which is the relay agent's interface IP). If the server does not have a route to that subnet, the OFFER will be dropped. The ping from R2 to R1 succeeds because R1's interface is directly connected, but the return traffic from R1 to the giaddr (which is on a different subnet) may fail if R1 does not have a route.

What should I do if I get this 300-410 question wrong?

Are there clue words in this question I should notice?

Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

What is the key concept behind this question?

CIDR notation defines the prefix length.

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Last reviewed: Jun 19, 2026

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