How do I enable an interface to hand out IPs on a certain subnet on a Cisco switch?

Question

I have telnet access to a Cisco switch, only I need to enable an interface so it can start handing out IPs on a certain subnet. What commands do I need to use to get to the interface config file or area? I had another sysadmin show me once, but he has since moved on and I inherited his setup. I've been going over the iOS manuals, but I can't see the way to do this. This is probably a really basic question, so I apologize if I'm just wasting electricity.

Answer 1

You're looking for a DHCP server, which is a functionality that is not provided by the switch.

What you will find in the IOS documentation - and possibly confuse you - is the possibility to act as a DHCP proxy between the requesting station and the server (this is useful in large networks since DHCP uses many broadcasts).

Answer 2

If you just wish to direct DHCP request on VLANs on the switch to a DHCP server on another VLAN you can use the ip helper-address feature.

Example if your DHCP server is 192.168.0.1 and the switch is the default gateway on Vlan10.

interface Vlan10
 ip address 192.168.10.254 255.255.255.0
 ip helper-address 192.168.0.1
end

Answer 3

If your Cisco device really is a plain switch, not a router/multilayer switch then it cannot, by definition, be a DHCP server. However, multi-layer switches can, this is an excerpt from a Cisco page on the subject:

The Cisco IOS DHCP Server feature is a full DHCP Server implementation that assigns and manages IP addresses from specified address pools within the router to DHCP clients. If the Cisco IOS DHCP Server cannot satisfy a DHCP request from its own database, it can forward the request to one or more secondary DHCP Servers defined by the network administrator.

You can set up a simple DHCP service with the command "ip dhcp pool", there's a few other commands to configure do the full configuration but that will get you started.

Answer 4

As said before - it much depends on the exact switchtype and the licensed features - but most Cisco L3-switches are able to hand out IP-adresses via DHCP.

See Cisco IOS Configuration Guide - DHCP for further information.

But beware: This feature is only needed on one switch per VLAN (if it spans over a couple of switches) - so be shure to find the right one :-)

tsg