DHCP Relay for a Remote DHCP Server on a New Subnet
Short answer
A Configure an IP helper address on the new segment's router or Layer 3 switch so DHCP broadcasts are relayed as unicast to the remote DHCP server.
Question
Key Takeaways
It tests whether you recognize that DHCP DISCOVER is a local broadcast and that a Layer 3 device needs an IP helper to relay it to a remote server; the trap is assuming server-side scope or lease settings can fix a broadcast that never arrives.
This N10-009 question tests DHCP relay: when clients and the DHCP server sit on different subnets, an IP helper address on the default gateway forwards discover broadcasts to the server. The correct answer is A.
Common mistake: Choosing C (adjust the existing DHCP lease time) mistakes an initial-addressing failure for a lease-duration issue; lease time affects renewal after a client has a lease, not whether the first DISCOVER reaches a remote DHCP server.
Expert Analysis
Why the new segment fails DHCPv4 clients begin with no address and send DISCOVER as a Layer 2 broadcast to 255.255.255.255; routers do not forward that broadcast by default. Because the DHCP server is on a remote segment, the new subnet's clients can send locally but nothing reaches the server, so they fall back to APIPA. Option A is correct: an IP helper (DHCP relay) on the Layer 3 gateway forwards those requests.
What the helper changes, and the adjacent decisions The helper intercepts the client broadcast on the new VLAN's router interface or Layer 3 switch SVI and unicasts it to the DHCP server, typically setting the gateway address so the server selects the right scope for that subnet. The adjacent checks matter as much as the command: the client VLAN must have the helper configured, not just the server-facing link; routing must exist in both directions; and the DHCP server must have a scope, exclusions, and default gateway option for the new segment. If the server is local to the clients, no helper is needed, and the investigation shifts to scope exhaustion, reservations, or lease timers.
What would flip the answer If the scenario said the DHCP server had been moved onto the new segment, or clients on the same VLAN as the server were failing, A would no longer be the fix; then a wrong VLAN assignment, disabled switch port, or exhausted scope would be more likely. Likewise, if only renewals failed while new clients worked, lease-time or reservation issues would deserve attention. Option B is 802.1X port authentication, unrelated to DHCP. Option C changes how long an existing lease lasts, not whether the first DISCOVER crosses a router. Option D is harmful because DHCP clients and servers expect UDP 67 and 68.
Operational consequence Getting this wrong leaves the new segment without usable addressing and sends responders chasing server-side settings while the server sees no requests. The community explanation captures the key: DHCP discovery is a local Layer 2 broadcast that routers block, so the gateway needs an IP helper to relay it by unicast. A misplaced helper or missing return route creates the same outage, and unmanaged relay can expose clients to an unauthorized DHCP server, so pair it with DHCP snooping and trusted uplinks.
Official Reference
- https://www.comptia.org/certifications/network
- https://www.cisco.com/c/en/us/support/docs/ip/dynamic-address-allocation-resolution/27470-100.html
- https://www.rfc-editor.org/rfc/rfc2131
These links open the official vendor documentation in a new tab.
Exam Strategy
When a service works on the local segment but fails after a new subnet is added, map the protocol's traffic first: DHCPv4 uses broadcast, so check the Layer 3 gateway's relay setting before touching server scopes. Look for the word remote in the stem—it signals a crossing of broadcast domains. Eliminate options that alter server-side timers, authentication, or port numbers without addressing forwarding.
Frequently Asked Questions
Why can't the DHCP server hear clients on the new subnet?
Where should the IP helper address be configured?
Does changing DHCP lease time fix clients that never get a lease?
Community Votes
100% of anonymous learners picked answer A. Votes are pick records left by other test-takers — they are not the verified answer.
Discussion 1 contributions
Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.
From public community threads 1
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ERB14 👍 1 Answered AIP Helper (DHCP Relay): DHCP client discovery messages are sent as local Layer 2 broadcasts (255.255.255.255), which routers block by default and do not forward across subnets. Configuring an IP helper-address on the default gateway (router or Layer 3 switch) intercepts these broadcast requests and forwards them via unicast to the remote DHCP server.
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