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5.6. DHCP Client and Relay

đź’ˇ First Principle: DHCP uses broadcasts, but routers don't forward broadcasts. This creates a problem: how does a client on one subnet get an address from a DHCP server on a different subnet? DHCP relay (ip helper-address) solves this by converting the client's broadcast into a unicast directed at the DHCP server.

Consider this troubleshooting scenario: Users on a new floor report "no network connectivity." You check their laptops—they all have 169.254.x.x addresses. The DHCP server is working fine; other floors get addresses. What's different? The new floor is on a new VLAN with a new router interface. Someone forgot to configure ip helper-address. The clients are broadcasting requests, but those broadcasts never reach the DHCP server.

What happens without DHCP relay: A new laptop broadcasts "I need an IP address!" The local router receives this broadcast but drops it—routers don't forward broadcasts by design. The DHCP server on a central subnet never hears the request. The laptop falls back to APIPA (169.254.x.x), which means no routing, no connectivity beyond the local link.

Why centralized DHCP matters:
  • Single source of truth: One DHCP server for the whole network, not one per subnet
  • Easier management: Update DNS servers or lease times in one place
  • IP address tracking: Logs show who had what address when (critical for security)
DHCP Relay Configuration:
Router(config)# interface GigabitEthernet0/0
Router(config-if)# ip helper-address 10.1.1.100

The relay agent:

  1. Receives DHCP broadcast on the interface
  2. Adds its interface IP to the gateway IP address (giaddr) field
  3. Forwards as unicast to the helper address
  4. Server uses giaddr to select the correct scope