we got here 2 x 6509/Sup 720 running 12.2(18)SXD2. Some 2950 Access switches are connected via trunk (12.2(18)SXD2). We have a Novell DHCP/PXE Server running Netware 6.5.
On our core is a HSRP process running *.253 (core1) *.254 (core2) + virtual *.1
No dhcp spoofing configured. Spanning-tree porfast is enabled (access)
If a PC or Laptop is disconnected longer than 3 days the PXE boot fails and the client didn't get an IP address at the first attempt from the DHCP Server. If we turn off the client a few times the PXE boot request will be successful and we will get an IP address.
We checked the Novell log and we can see that the server is responding to the request from the client. The server is forwarding an IP address out of the DHCP pool to the client. The client accepts the IP and it's waiting for the ACK, the novell log entry shows "warning there is no matching entry configured for client with mac address, sending bootp /dhcp reply (dhcp NAK) to mac address of the client as *253 and *.254 (our core)
The DHCP server is sending out a new DHCP offer with a new ip address for the client to both cores *253+*254.
The client keeps the address that was discovered first and persist to use this address via a DHCP request to the server.
We had sniffed the traffic from the client and the server port as well and we can see that the incoming DHCP Discover. Then both Core switches with IP 253,254 doing a DHCP offer broadcast and starting the communication with the DHCP server. Now both Core switches are sending DHCP ACK broadcasts. The client communicates now directly with the DHCP Server. It's sending the ACK but 1second later it requests a DHCP release and the core Servers sending out a NAK and everything starts again from the beginning.