Cisco Certified Network Associate (CCNA) 200-301 · Free study guide

Objective 4.6 — Configure DHCP clients and relay

DHCP begins before a client has ordinary IP configuration, so its early messages use local broadcasts. Routers separate broadcast domains; a relay bridges that service workflow without bridging every broadcast.

DHCP client mode

A routed interface configured as a DHCP client requests an address and options from an available server or relay path. Verify the interface acquired an address, mask, lease, and any required gateway or resolver information. Client mode is different from operating a DHCP server pool.

An administratively down interface or an interface in the wrong VLAN cannot complete the lease process simply because client mode is configured.

DORA exchange

The familiar sequence is Discover, Offer, Request, and Acknowledgment. A client broadcasts discovery; servers can offer configuration; the client requests a selected offer; the server acknowledges the lease. Renewals can use unicast under normal conditions. Security controls such as DHCP snooping may filter server messages based on trusted directions.

Relay behavior

Configure the relay or helper on the Layer 3 interface that receives client broadcasts, pointing to the server's unicast address. The relay supplies gateway or subnet context so the server selects the correct scope and then returns the response toward the client network.

Putting a helper on the server-facing interface does not capture the client's incoming broadcast. Multiple helpers can support redundant servers, with server policy deciding offers.

IOS command map

On an IOS-style routed interface that should obtain its own IPv4 configuration, use ip address dhcp and ensure the interface is enabled. On the gateway for a remote client VLAN, use ip helper-address 192.0.2.60, substituting only the approved server address. The helper belongs on the interface that receives the client broadcast, not on an arbitrary server-facing port.

Use show dhcp lease where supported to inspect an interface client's lease and show ip interface brief to confirm its assigned address and state. For relay, inspect show running-config interface Vlan20, routing to the server, the server scope, and DHCP snooping state. A controlled packet capture or server log can identify whether Discover, Offer, Request, or Acknowledgment is the first missing stage. Command output varies by IOS platform.

Worked scenario

Clients in VLAN 20 fail, while clients on the server's VLAN succeed. The VLAN 20 gateway interface has no helper. Configure the approved server address on that gateway, confirm routing both ways, and verify the server has a VLAN 20 scope. If offers arrive but clients never accept them, inspect snooping trust, VLAN, and return handling rather than adding more helpers at random.

Verification stack

Inspect client lease state, gateway interface and helper, server scope and available addresses, routing, ACLs, DHCP snooping, and logs. A packet capture can show which DORA phase stops. For a device interface acting as client, inspect its lease and installed connected or default information as appropriate.

Common traps

• Expecting routers to forward DHCP broadcasts automatically.
• Configuring relay on the wrong interface.
• Forgetting the server scope for the remote subnet.
• Treating a helper as a DHCP server.
• Ignoring DHCP snooping trust and bindings.

Readiness checklist

• I can configure an interface as DHCP client.
• I can place relay on the client-facing gateway.
• I can narrate DORA and relay context.
• I can localize the failed exchange phase.