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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.

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which switch is it? Catalyst XXXX? –  Edgar Velasquez Lim Oct 29 '11 at 13:08

4 Answers 4

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).

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Nope, not a DHCP server, just need to know how to enable the interface. I already have a DHCP server. I'll post the model number by Monday, but I thought there would be a well-known way to get to an interface config file area. –  user65712 Oct 30 '11 at 11:46

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
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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.

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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

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