Controlling DHCP Broadcasts on a LAN

mcrockett · ‎02-18-2008

Scenario 1: You have a conference room that is configured to support meetings of users on segregated networks each with a DHCP server. Manager fiddles with ethernet cable connected to a port in the conference table. Absent-mindedly connects the other end of the cable to another port that connects to the other network.

Scenario 2: Engineer needs to prepare for a trip to a customer site. Needs to update a second laptop that he will be taking with him. Brings a DHCP-capable Linksys hub from home and connects the corporate LAN to the wrong port.

In both instances, you have DHCP requests being broadcast to the wrong DHCP server and have users unable to access resources because their DHCP client is assigned an IP address from the wrong network.

I ran across a Cisco article on this some time agon but haven't been able to find it again. I would appreciate links to the article or clues on constructing a search request to find it.

One thing that I do recall about the article is that it was a global configuration option which might present a problem when you only want to apply this to a single VLAN.

Helpful hints are most welcome.

Edison Ortiz · ‎02-18-2008

The feature you are looking for is called DHCP Snooping. It's available in most Cisco switches. Here is some information on how it's done in a 4500 switch:

http://www.cisco.com/en/US/docs/switches/lan/catalyst4500/12.1/13ew/configuration/guide/dhcp.html

HTH,

__

Edison.

mcrockett · ‎02-18-2008

The core switch at this site happens to be a 4500 Series. How did you know? :)

After a quick read of the article, I have the impression that DHCP Snooping must be enabled on all switches connected to the 4500. Would the trunk port be declared as trusted or untrusted?

Edison Ortiz · ‎02-18-2008

The only trusted interfaces must be the ones connected to the DHCP server and inter-switch links.

From the link I posted:

DHCP snooping acts like a firewall between untrusted hosts and DHCP servers. It also gives you a way to differentiate between untrusted interfaces connected to the end-user and trusted interfaces connected to the DHCP server or another switch.

HTH,

__

Edison.