IP Services // Lesson 06

DHCP Client & Relay.

Configure an IOS interface as a DHCP client and relay client broadcasts to a server on another subnet.

DHCP clientRelay agentHelper addressBroadcastGateway interface
01 // Mental model

Start with the big picture.

A router does not normally forward the client's local DHCP broadcast to another subnet.

CCNA focus: Place the helper on the interface where client broadcasts arrive—not on the server-facing interface. Then verify server reachability, the matching scope and relay counters/debugs.
02 // Building blocks

Know what each part does.

01

Broadcast boundary

A router does not normally forward the client's local DHCP broadcast to another subnet.

02

Relay

The gateway interface receives the broadcast and unicasts a relayed request to the configured server.

03

Gateway information

The relay identifies the client subnet so the server can select the correct address pool.

04

IOS client

An interface can obtain its own IPv4 address dynamically with ip address dhcp.

03 // Compare and recognise

Read the clues.

ItemWhat to remember
ip helper-addressConfigured on the client-facing Layer 3 interface, pointing to the DHCP server.
ip address dhcpMakes the IOS interface a DHCP client.
Server routeThe DHCP server and relay need working return-path routing.
Pool scopeThe server needs a matching scope for the relayed client subnet.
04 // Configure and verify

Configure and verify.

R1(config)# interface gigabitEthernet 0/0
R1(config-if)# ip helper-address 10.50.0.10

EDGE(config)# interface gigabitEthernet 0/1
EDGE(config-if)# ip address dhcp
EDGE(config-if)# no shutdown

R1# show ip interface gigabitEthernet 0/0
EDGE# show dhcp lease

Read the example from top to bottom, then verify the resulting state. Configuration is only complete when the output matches the intended design.

05 // Exam and troubleshooting

Turn facts into a method.

  • Configured on the client-facing Layer 3 interface, pointing to the DHCP server.
  • Makes the IOS interface a DHCP client.
  • The DHCP server and relay need working return-path routing.
  • The server needs a matching scope for the relayed client subnet.
Exam checkpoint: Place the helper on the interface where client broadcasts arrive—not on the server-facing interface. Then verify server reachability, the matching scope and relay counters/debugs.
06 // Check yourself

DHCP Client & Relay quiz.

1. Where is ip helper-address configured for a client VLAN?

2. Why is a relay needed across subnets?

3. Which command makes an IOS interface a DHCP client?

4. How does the server choose the right scope for a relayed request?

5. What else must exist between relay and server?

Score: 0 / 5