hardMultiple Choice
300-410 Practice Question: An engineer configures DHCPv4 on a router with…
An engineer configures DHCPv4 on a router with multiple pools for different subnets. Clients in subnet A receive addresses correctly, but clients in subnet B receive addresses from subnet A's pool. The router has 'ip dhcp relay' configured. Which is the most likely explanation?
⚠ Common exam trap
Cisco often tests the misconception that DHCP pool selection is based on pool order or client MAC address, when in fact it relies on the giaddr set by the relay agent.
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 selects the pool based on the giaddr; if the relay agent does not set the giaddr correctly, the server may use the first matching pool.
When a DHCP relay agent forwards a client's request to the server, it inserts its own IP address (the interface address on the client's subnet) into the giaddr (gateway IP address) field. The DHCP server uses this giaddr to select the appropriate pool. If the relay agent fails to set the giaddr correctly—for example, due to misconfiguration or the relay interface not being on the correct subnet—the server may receive a giaddr of 0.0.0.0 or an incorrect address, causing it to fall back to the first configured pool that matches, which in this case is subnet A's pool.
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 selects the pool based on the giaddr; if the relay agent does not set the giaddr correctly, the server may use the first matching pool.
Why this is correct
The DHCP server's pool selection for relayed requests hinges on the giaddr field. When a client broadcast is relayed, the relay agent must set giaddr to the IP address of the interface facing the client, and the server then matches that address against the network statements of its pools. If the giaddr is missing or set incorrectly—for example, because 'ip helper-address' is applied on the wrong interface or omitted—the server cannot identify subnet B and may select the first pool whose network range loosely matches, resulting in subnet A addresses.
- ✗
The pools are configured in the wrong order; the server uses the first pool that matches the client's MAC address.
Why it's wrong here
DHCP pool selection is not a first-come, first-served MAC match. The server chooses a pool by comparing the giaddr or the receiving interface's subnet to the pool's network statement; MAC addresses are used only for manual reservations (host statements), not for ordering. Even if pools were listed in a particular order, the server does not iterate pools by client MAC, so reordering pools cannot change which pool is chosen for a relayed client.
- ✗
The router has 'ip dhcp smart-relay' enabled, which overrides pool selection.
Why it's wrong here
The 'ip dhcp smart-relay' feature does not override or alter the pool-selection logic; it simply enables the router to insert a giaddr for broadcast DHCP packets on interfaces without an 'ip helper-address' configuration, using the interface IP as the gateway address. Instead of causing the wrong pool, smart relay would actually improve the chance that the server receives a correct giaddr and selects the proper pool. Thus, this option misidentifies the feature's purpose.
- ✗
The DHCP server is configured with 'network' statements that overlap.
Why it's wrong here
Overlapping 'network' statements in DHCP pools are a separate misconfiguration that can produce pool conflicts or server startup errors, and they are generally rejected or cause address clashes, not a clean handoff of subnet B clients to subnet A's pool. The symptom described is a relay/giaddr failure scenario, where the server sees no valid giaddr and falls back to the first available pool. Overlapping pools would not change how giaddr is processed, so they cannot be the root cause here.
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Related to this question
Learn chapter
Infrastructure Services: DHCP, NAT, and IPv6
Key term
DHCP Client and Server
A DHCP client is any device that requests network configuration settings from a DHCP server, which automatically assigns IP addresses and other parameters to devices on a network.
Key term
DHCP Relay Configuration
A DHCP relay configuration allows a router to forward DHCP broadcast requests from clients on one subnet to a DHCP server on a different subnet, enabling centralized IP address management across multiple networks.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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