We have 2 DHCP servers in the network both running in a Active- Passive cluster. And we recently started getting some event logs "DHCPerr1003 No managed subnets/scopes"in the location "C: sysmgt csc cfg dhcplog1.txt "

The problem is the two DHCP servers on the same subnet Here is the MS explanation. It sounds like you are running your DHCP servers off a Windows server?
You can do this type of stuff off Cisco equipment in a VLan environment and it is foten used to do VOIP, or wireless VLans.
Good luck I can't help much more being that I run Linux servers and a Cisco Network and do all my DHCP via the a network device not the server.