PCs not finding finding network after reboot

[cas8099]

After much googling and many rabbit holes, I've decided to come here. I'm helping out someone who took over a small network. Randomly, when systems reboot the NICs don't connect back to the domain network "domain.local" but to the generic "Network 3", which equates to no connectivity. If I rebooted all 25 Windows 7 machines probably 3 or 4 random ones will do this. After disabling and enabling the NIC, sometimes more than once, it'll connect to the domain network. The router is a 2901 and the switch is a 2950. Obviously my Cisco knowledge is seriously lacking and any help would be greatly appreciated. 

[Georg Pauwen]

Hello,

on the switchports (the interfaces of the 2950 switch) make sure the command 'spanning-tree portfast' is configured.

Is it possible to post the running configs (show run) of both the router and the switch ?

[Joseph W. Doherty]

The "why" being what Georg posted, Windows user PCs often are set to obtain DHCP IPs. Cisco switches, by default, block full port usage if STP is running (which by default they generally do), until STP confirms fully enabling the port will not create a L2 loop. If the STP loop check takes longer than the host is willing to wait for a DHCP address, it may just generate its own IP, which generally precludes that host not working with other hosts.

The "spanning-tree portfast", on host ports (or globally enabled), "short circuits" STP from confirming this port might create a L2 loop, ane so the port is enabled for full operation much, much faster.

[Leo Laohoo]

Sounds the the PCs are not getting a valid IP address.

[Kevin Martin]

You might look at IP Device Tracking Probe Delay:

https://www.cisco.com/c/en/us/support/docs/ios-nx-os-software/8021x/116529-problemsolution-product-00.html