In some cases, we need a router to be configured as DHCPv4 clients just like a client computer. But this method is not recommended for routers in an internal network. The dynamic addressing is extremely useful for routers that connect to the Internet through an ISP. We can simply connect a router using just the plug-in Ethernet cable. To configure an Ethernet interface as a DHCPv4 client, we use the ip address dhcp command in interface configuration mode.
When an interface on the router is configured with ip address dhcp command, it can dynamically learn its IP address and network mask via DHCP. The router can also learn its TFTP server address, NETBIOS name server, DNS servers, vendor-specific information, static routes, domain name, and default router information.
Configuration
The figure below contains the topology, including DHCPSERVER and Client. You may suppose that the DHCPSEVER is an ISP server configured to provide DHCP service to clients with IP addresses from the selection of 192.168.1.2 – 192.168.1.100.
After the Fa0/0 interface of the client, the router is configured with the ip address dhcp command. We can verify the configuration using the show ip interface fa0/0 command, which confirms that the interface is up and that the address was allocated by a DHCPv4 server. Now let’s come to the configuration:
DHCP Server Configuration
DHCPSERVER> enable DHCPSERVER# configure terminal Enter configuration commands, one per line. End with CNTL/Z. DHCPSERVER(config)# ip dhcp excluded-address 192.168.1.1 DHCPSERVER(config)# ip dhcp pool MY-POOL DHCPSERVER(dhcp-config)# network 192.168.1.0 255.255.255.0 DHCPSERVER(dhcp-config)# default-router 192.168.1.1 DHCPSERVER(dhcp-config)# dns-server 8.8.8.8 8.8.4.4 DHCPSERVER(dhcp-config)# domain-name networkustad.com DHCPSERVER(dhcp-config)# lease 0 12 DHCPSERVER(dhcp-config)# exit DHCPSERVER(config)# interface FastEthernet0/0 DHCPSERVER(config-if)# ip address 192.168.1.1 255.255.255.0 DHCPSERVER(config-if)# no shutdown DHCPSERVER(config-if)# exit DHCPSERVER(config)# exit DHCPSERVER# write memory Building configuration... [OK]
| Command | Purpose |
|---|---|
excluded-address | Prevents IP conflict with router |
dns-server | Provides Google DNS to clients |
domain-name | Appended to unqualified hostnames |
lease 0 12 | 12-hour lease (good for labs) |
DHCP Client Configuration
Client> enable Client# configure terminal Enter configuration commands, one per line. End with CNTL/Z. Client(config)# interface FastEthernet0/0 Client(config-if)# ip address dhcp Client(config-if)# no shutdown Client(config-if)# end Client# write memory Building configuration... [OK]
Note: The router sends a DHCP Discover → receives Offer → sends Request → gets ACK.
Verify DHCP Client Configuration
Now check and verify the address configuration using the show ip interface fa0/0 command. The figure below illustrates the output of this command.
Client# show ip interface FastEthernet0/0 FastEthernet0/0 is up, line protocol is up (connected) Internet address is 192.168.1.3/24 Broadcast address is 255.255.255.255 Address determined by DHCP MTU is 1500 bytes Helper address is not set Directed broadcast forwarding is disabled Outgoing access list is not set Inbound access list is not set Proxy ARP is enabled Security level default Split horizon is enabled ICMP redirects are always sent ICMP unreachables are always sent ICMP mask replies are never sent IP fast switching is enabled IP CEF switching is enabled IP Flow switching is disabled IP multicast fast switching is enabled IP multicast distributed fast switching is disabled IP route-cache flags are Fast, CEF Router Discovery is disabled IP output packet accounting is disabled IP access violation accounting is disabled TCP/IP header compression is disabled RTP/IP header compression is disabled Probe proxy name replies are disabled Policy routing is disabled Network address translation is disabled BGP Policy Mapping is disabled
View DHCP Lease Details
Client# show dhcp lease Temp IP addr: 192.168.1.3 for peer on interface: FastEthernet0/0 Temp sub net mask: 255.255.255.0 DHCP Lease server: 192.168.1.1 DHCP remaining lease time: 11:59:23 Renewal (T1) time: 6:00:00, Rebind (T2) time: 10:30:00
Confirm Default Gateway
Client# show ip route | include Gateway Gateway of last resort is 192.168.1.1 to network 0.0.0.0
Troubleshooting DHCPv4 Client Issues
| Symptom | Command | Solution |
|---|---|---|
| No IP assigned | debug ip dhcp client | Check physical link, server reachability |
| Interface down | show ip interface brief | Run no shutdown |
| Wrong subnet | show dhcp lease | Verify server pool matches network |
| Lease expired | show clock | Force renewal |
| Conflict detected | show ip dhcp conflict | Clear with clear ip dhcp conflict * |
Force Release & Renew
Client# release dhcp FastEthernet0/0 Client# renew dhcp FastEthernet0/0
DHCPv4 Client vs Static IP on Router
| Feature | ip address dhcp | Static IP |
|---|---|---|
| Auto-config | Yes | No |
| ISP WAN | Ideal | Manual setup |
| Internal LAN | Not recommended | Preferred |
| DNS/Gateway | Auto-learned | Manual config |
| CCNA Focus | Exam topic | Core skill |
| Reliability | Depends on server | Always up |
Use Case Examples:
- Branch office WAN interface
- SOHO router connected to cable/DSL modem
- Lab testing (Packet Tracer, GNS3)
- Temporary migration scenarios
CCNA Exam Tip: The ip address dhcp command is part of the IP Services domain in CCNA 200-301. You must know how to configure and verify it.
Warning: Not recommended for internal LAN-facing interfaces — use static IPs or configure the router as a DHCP server.
FAQs
What command configures a router interface as a DHCPv4 client?
Use ip address dhcp in interface configuration mode (e.g., interface fa0/0). The router sends DHCP Discover and receives IP, subnet mask, default gateway, and optional parameters like DNS from the server.
How do you verify a router received an IP via DHCP?
Run show ip interface [interface] (e.g., show ip interface fa0/0). Look for “Internet address is X.X.X.X/24” and “Address determined by DHCP” to confirm dynamic assignment and up/up status.
Can a Cisco router learn more than just an IP from DHCP?
Yes. With ip address dhcp, it can receive default gateway, DNS servers, domain name, TFTP server, NETBIOS, vendor-specific options, and even static routes from the DHCP server.
When should you configure a router as a DHCP client?
Best for Internet-facing (WAN) interfaces connecting to ISPs. Not recommended for internal LAN routers where static IPs ensure reliability and control over addressing.
What does the DHCP server need to exclude in the pool?
Exclude the router’s own IP (e.g., ip dhcp excluded-address 192.168.1.1) to prevent conflicts. Then define the pool with network, default-router, and other options.
