DHCP Client and Relay Configuration

What it is

Two configuration jobs involve DHCP, in opposite roles.

A DHCP client is an interface that asks for its own address from a server. This is common on the internet-facing interface of a small office router, where the provider supplies the address.

A DHCP relay helps hosts reach a server that is not on their own subnet. Hosts find a server by broadcasting, and a router does not forward broadcasts by default. The relay catches the broadcast and passes it on as a unicast packet.

The earlier DHCP and DNS topic explained Discover, Offer, Request, Acknowledge. This page builds on it.

How it works

The client sends a broadcast, and the relay turns it into a unicast to the server.

Interface as a DHCP client

Replace the static address with one word:

interface GigabitEthernet0/1
 ip address dhcp
 no shutdown

The interface runs the same four-step conversation a host would. If it succeeds, it gets an address and mask, and may learn a gateway and DNS server if the server provides them.

If the server supplies a default router option, the interface installs a default route from it. That route carries an administrative distance of 254 by default, so any other default route wins.

To verify:

show ip interface brief
show dhcp lease

• In show ip interface brief, the Method column shows DHCP for a learned address, and manual for a typed one.
• An interface with no address shows unassigned.
• show dhcp lease displays the lease details.

On the server side, show ip dhcp binding lists handed-out addresses, and show ip dhcp pool summarises each pool.

DHCP relay

Picture a host on VLAN 10 and a DHCP server on a different subnet.

The command goes on the router interface that faces the hosts:

interface GigabitEthernet0/0
 ip address 192.168.10.1 255.255.255.0
 ip helper-address 10.20.20.5

Here is the sequence:

1. The host broadcasts a Discover.
2. The interface with ip helper-address receives it.
3. The router converts it to a unicast addressed to 10.20.20.5.
4. The router writes its own interface address into the gateway address field of the message.
5. The server reads that field to learn which subnet asked, and picks an address from the matching pool.
6. The reply returns through the relay to the host.

The gateway address field is the key idea. It lets one server run several pools and choose the right one for each remote subnet.

More than DHCP

ip helper-address also forwards other UDP broadcast services by default. These are time (37), TACACS (49), DNS (53), TFTP (69) and the NetBIOS name and datagram services (137 and 138), plus the IEN 116 name service (42).

You can switch individual services off with no ip forward-protocol udp PORT, but this objective focuses on DHCP. You can also configure several helper addresses on one interface, and the router relays to each.