DHCP

Overview

DHCP automates IP configuration (address, mask, gateway, DNS) so clients can join a network without manual static settings. CCNA focuses on DORA, router pools, ip helper-address relay, and how DHCP ties to voice VLAN and WLC AP discovery.

What DHCP provides

DHCP (Dynamic Host Configuration Protocol) is a client/server protocol that automatically assigns hosts an IP address, subnet mask, default gateway, DNS servers, and optional parameters (domain name, NTP, TFTP for IP phones, etc.). Centralized assignment reduces manual errors, address conflicts, and admin workload.

The DORA process

When a DHCP client boots, it follows DORA:

Lease lifecycle

• Clients receive a lease with an expiration time
• Before expiry, the client sends a DHCP Request to renew (often at 50% and 87.5% of lease time)
• If renewal fails, the client may release and restart DORA

Who should use DHCP?

Servers and infrastructure devices are not typical DHCP clients — their stability depends on fixed addresses.

Cisco router as DHCP server

A Cisco router can act as a DHCP server for directly connected subnets using an ip dhcp pool.

ip dhcp excluded-address 10.10.10.1 10.10.10.10
!
ip dhcp pool LAN-CLIENTS
network 10.10.10.0 255.255.255.0
default-router 10.10.10.1
dns-server 10.10.20.10
domain-name example.com
lease 7

show ip dhcp pool
show ip dhcp binding
show ip dhcp conflict
clear ip dhcp binding *

DHCP relay (ip helper-address)

DHCP broadcasts do not cross routers. When the DHCP server is on a different subnet than clients, configure a DHCP relay agent on the router interface facing the clients.

interface FastEthernet0/1
description User VLAN 10
ip address 10.10.10.1 255.255.255.0
ip helper-address 10.10.20.10

The router receives client broadcasts, unicasts them to the DHCP server IP, and relays responses back. Without relay (or a server on every subnet), remote subnets cannot obtain leases.

Relay topology (exam mental model)

[PC VLAN 10] --broadcast DORA--> [R1 Fa0/1 + helper-address]
                                        |
                                   unicast to
                                        |
                              [DHCP Server 10.10.20.10]

Cisco router as DHCP client

ISPs often assign a dynamic public IP via DHCP on the WAN interface. PAT overload then shares that single address for inside hosts.

interface FastEthernet0/0
ip address dhcp
no shutdown
!
show dhcp lease

Common pattern: outside interface gets DHCP public IP → ip nat inside source list 1 interface Fa0/0 overload for inside users.

DHCP options (awareness)

Beyond IP/mask/gateway/DNS, DHCP options carry extra data:

Voice VLAN / CUCME scenarios often need DHCP option 150 — see the Voice VLAN topic. Option 43 on the DHCP server helps lightweight APs find a WLC — see Wireless Configuration.

DHCPv6 (brief cross-reference)

IPv6 hosts can get addresses via SLAAC (stateless) or DHCPv6 (stateful). DHCPv6 relay uses ipv6 dhcp relay destination on interfaces. See the IPv6 topic for SLAAC vs DHCPv6.

DHCP snooping (security context)

A rogue DHCP server on the LAN can hand out malicious gateway/DNS addresses (MITM). DHCP snooping on switches builds a binding table of legitimate leases and drops unauthorized DHCP offers.

Troubleshooting workflow

1. Confirm the client interface is set to obtain IP automatically
2. On the client: ipconfig /release and ipconfig /renew (Windows) or dhclient (Linux)
3. On the router: show ip dhcp pool, show ip dhcp binding
4. For relay scenarios: verify ip helper-address points to the correct server IP
5. Check ip dhcp excluded-address — is the gateway accidentally in the pool?
6. Verify ACLs are not blocking UDP 67/68 between client, relay, and server

Exam checklist