A centralized DHCP server provides network configuration from one managed service instead of having each LAN router serve its own clients. Centralization simplifies DHCP-pool management, keeps policies consistent, reduces the number of servers, and can provide features beyond Cisco IOS. DHCP leases clients IP addresses and other settings such as the netmask, default gateway, and DNS servers, while maintaining state about active leases. section 4.2 section 4.1
A centralized deployment depends on these operational facts:
- DHCP uses the four-message DORA exchange: the client broadcasts DISCOVER, the server sends OFFER, the client broadcasts REQUEST, and the server confirms with ACK. section 4.1.1
- DHCP uses UDP port 67 for servers and UDP port 68 for clients; the terminology follows RFC 2131. section 4.1.1
- Routers do not forward DHCP broadcasts between subnets, so clients on remote LANs require a DHCP relay agent. section 4.2
- The relay receives client DISCOVER and REQUEST broadcasts, forwards them as unicast packets to the centralized server, and returns OFFER and ACK messages to clients. section 4.2
Configure a Cisco IOS relay
Configure the helper address on the router interface that receives the clients’ broadcasts: R1(config)# interface g0/0; R1(config-if)# ip helper-address 10.2.2.10. Verify the placement with show ip interface g0/0 and confirm that the output reports Helper address is 10.2.2.10. The command belongs on the client-facing interface, not merely any interface on the router. section 4.2
