It kinda disturbs me that you mention your “core switch” in the same thought with wanting to set it up as a DHCP server. The core of your network should be concerned with nothing but moving things around as fast as possible. And you want to now assign a new task to that switch as well.

Judging by the fact that you mentioned “a VLAN” in the singular, I assume that you’re a relatively small organization without very many users/machines to assign addresses to. So we’ll work under that premise where the processing power is still within the realm of sanity.

So now, onto the configuration! Everything with the DHCP server configuration is done in global mode (IOS-based switches). If your network uses 172.27.5.1 to 172.27.5.254for its host addresses, then the configuration would be:

Switch(config)#ip dhcp pool CoreNet
Switch(dhcp-config)#network 172.27.5.0/24
Switch(dhcp-config)#default-router 172.27.5.1
Switch(dhcp-config)#dns-server 172.27.5.10 172.27.5.20 172.27.5.30
Switch(dhcp-config)#netbios-name-server 172.27.5.15 172.27.5.25
Switch(dhcp-config)#domain-name mycore.net

That enables your switch to start responding to DHCP broadcasts to the network shown in the network command. It will specify the default gateway (default-router) to be used by the hosts, as well as a domain name, DNS servers and WINS servers.

It’s also a good idea to exclude pre-assigned addresses from the pool. That’s a separate set of global commands, and not a pool configuration option. Notice that the servers listed above top out at .30. Hopefully, in the design of your network, there’s already a set delineation between “server IP addresses” and “host IP addresses.” This is a good design practice to make your life easier in the long-term maintenance of the network.

Switch(config)#ip dhcp excluded-address 172.27.5.1 172.27.5.35

One of the bad reasons for using your switch as the DHCP server is that it assigns addresses in a sequential order, but really does nothing in terms of keeping a database by default. So if the power recycles on your switch for any reason, it will start assigning addresses out at the very beginning of the pool again.

For this, we like the switch to ping addresses before assigning them to a client. You may determine how many ping packets are sent out. The timeout value is in milliseconds.

Switch(config)#ip dhcp ping packet 5
Switch(config)#ip dhcp ping timeout 500

In order to see if anything goes bad, I would also recommend logging the information about IP address conflicts.

Switch(config)#ip dhcp conflict logging

Now, with that all being said, it’s a much better decision to place a small machine on your network to be the DHCP server in a “real” fashion. DHCPD is a free service within the Unix operating system (many of them also free). There’s a DHCP server that comes with Windows NT Server (not very robust) and Windows Server 2000/2003 (much better).

Due to the fact that other servers (without paying any more money) do a much better job at processing under the load of DHCP requests, like when users all come in at the beginning of business hours, it’s recommended that you get a service that will work better. Purpose-built DHCP servers will also maintain a database that survives power cycles. They allow reservations by MAC address, and many other options to certain machines (servers?) continue to receive the same IP assignment all the time no matter the reboot status of the server.

Think about these options during your network implementation. But ultimately, yes, it’s certainly possible to have your switch act as the DHCP server.

Hope this helps.
-- Scott

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Scott Morris, quadruple CCIE, JNCIE and all-around uber-geek, can often be seen traveling around the world consulting and delivering CCIE training. He recently accepted a new Senior CCIE Instructor position with Internetwork Expert! For more information on him check out http://www.uber-geek.net or for CCIE training check out http://www.internetworkexpert.com. You can contact Scott via editor@tcpmag.com. You can contact Scott about "Let's Add DHCP Services to the VLAN" at editor@tcpmag.com.