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.
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.
[toc:faq]The ProblemOn traditional switches whenever we have a trunk
interface we use the VLAN tag to demultiplex the VLANs. The switch needs
to determine which MAC Address table to look in for a forwarding
decision. To do this we require the switch to do...
[toc:faq]Introduction:Netdr is a tool available on a RSP720, Sup720 or
Sup32 that allows one to capture packets on the RP or SP inband. The
netdr command can be used to capture both Tx and Rx packets in the
software switching path. This is not a substitut...
IntroductionOSPF, being a link-state protocol, allows for every router
in the network to know of every link and OSPF speaker in the entire
network. From this picture each router independently runs the Shortest
Path First (SPF) algorithm to determine the b...