Another Catalyst Question

The other day, we find that a person is unable to get DHCP. People who already have a lease can do a renew just fine, but a MAC that has not previously been assigned an IP can not get one.

I find out that the cause is the result of Gratuitous ARPs. Apparently the Gratuitous ARP is an ARP meant to advertise that an IP is taken to prevent conflicts. When our DHCP server (A Cisco Router) sees this Gratuitous ARP, it will not hand out that IP anymore. I can do a “clear ip dhcp conflict *” to reset this behavior, but am finding that the table is being repopulated quickly. Apparently someone is spaming these ARPs.

The question is, is there a way to have this ARP type blocked on the switch?

Not sure how to turn off that feature, but it sounds like a scan on your network…Could be a virus, or it could be intentional.

Try using Wireshark and look for an abnormal number of ARP requests from a single IP, that may be your culprit.