Does this MAC present itself all the time, or just during boot? Marvel makes a NIC prevalent in some Dell systems, that presents MAC 0c00.0000.0000 during its startup process. If you run port security, and several people boot their computer within the cam table expiration period, port security will disable the port. You can work around it but it is time consuming in large networks where port security are enabled. Robert D. Scott Robert@ufl.edu Senior Network Engineer 352-273-0113 Phone CNS - Network Services 352-392-2061 CNS Receptionist University of Florida 352-392-9440 FAX Florida Lambda Rail 352-294-3571 FLR NOC Gainesville, FL 32611 321-663-0421 Cell -----Original Message----- From: Robert E. Seastrom [mailto:rs@seastrom.com] Sent: Friday, September 05, 2008 10:33 AM To: nanog@nanog.org Subject: [Fwd:] Nvidia NICs with duplicate mac addresses Forwarded to NANOG in the interests of wider awareness... having been there and torn out my already scarce hair, duplicate MAC addresses can really mess up your day... --- Just when you thought this couldn't happen any more... Copying from a different email list... mac address 04:4b:80:80:80:03, was showing up in multiple places across the network. I googled the mac address and discovered that other people are having the same issue with this mac address. Below are some links describing the problem: http://forums.nvidia.com/index.php?showtopic=22148 http://www.nvnews.net/vbulletin/archive/index.php/t-73469.html I just wanted everyone to know about this problem in case you run across similar slow "connectivity" issues. I believe the network card is made by NVIDIA.