We deployed DHCP snooping about 8 months and have found that mobile Windows 95 / 98 / NT 4.0 machines would fail getting IP addresses from the DHCP server if they had been on a previous subnet.
The reason this was happening was that the host would issue a DHCP request ( renewing an address from another subnet ) to the server - however the return NAK from the DHCP server would be suppressed by the access switch. See BUG CSCee84918
Unfortunately Windows 95 / 98 / NT 4.0 machines will issue ( 3 ) DHCP requests and if no NAK is recieved by the DHCP server - the client will keep the old ip address - or will fall back to 169.254.x.x. We found that XP and 2000 clients are smart enough to issue a DHCP DISCOVER after ( 3 ) requests and no NAK back from the DHCP server. However we have seen some XP and 2000 clients occasionally fail to get IP addresses as the result of the access switch suppressing NAK's.
We were unable to confirm from CISCO when this was fixed on the Catalyst 4500 - The bug indicates that this was resolved on the 3500 - but nothing on the 4500 -
We conducted testing ourselves with the latest 4K 122-25.EW code and are happy to report this was finally fixed. The NAK is only allowed back on the port in which client is making the request.
