Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 
Community Member

DHCP Binding Entries - Two IP addresses for same MAC


I've configure DHCP on the 3560 switches and noticed that it is causing a lot of havoc on my NAC.

When I display the binding table, I noticed a leading 01 attached to the MAC address and the same MAC has two entries.

See the following example:

SW004-01#sh ip dhcp binding | include fb         001c.259b.fb10          Jan 04 2012 01:58 PM    Automatic         0100.1c25.9bfb.10       Jan 04 2012 01:59 PM    Automatic         b8ac.6f80.e960          Dec 27 2011 02:13 AM    Automatic         01b8.ac6f.80e9.60       Dec 30 2011 10:11 AM    Automatic        0024.e843.f644          Jan 04 2012 09:03 AM    Automatic        0100.24e8.43f6.44       Jan 04 2012 09:03 AM    Automatic


I can not PING IPs mapped with the 12 digts MACs, but can PING IPs mapped to 14digts MACs (ARP still shows entries to 12 digts MACs).

What is happenting, the NAC receives a packet,, tries to connect to that IP to validate the host, but fails to connect as the host now has a new IP address for It is causing massive false-positives on the NAC side.

Why is this happeing on the switch?

Two IP addresses are being leased to same host, and this will quickly exhaust my pool.

Is there a fix?


CreatePlease to create content