Computer getting Unable to Identify Network UNLESS Vlan is changed

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A server stack is the collection of software that forms the operational infrastructure on a given machine. In a computing context, a stack is an ordered pile. A server stack is one type of solution stack — an ordered selection of software that makes it possible to complete a particular task. Like in this post about Computer getting Unable to Identify Network UNLESS Vlan is changed was one problem in server stack that need for a solution. Below are some tips in manage your windows server when you find problem about windows, networking, cisco, vlan, .

Question moved from Network Engineering to Server Fault.

This is a problem that has long plagued this environment.
At seemingly random times (measured generally in months), a computer may suddenly lose connectivity to the network.

It still is able to send packets through, reach and get an IP from the DHCP server, but it will simply fail to identify the network and display the yellow ! icon.

All the typical fixes – rebooting PC/port/switch, switching from DHCP to static or vise versa, changing ports, resetting NIC/winsock, reinstalling driver, etc. None of them will work.

But somehow, changing the access VLAN of the port will suddenly allow it to work. And seemingly nothing else. It doesn’t have to be a VLAN it was on before, or even one that existed on the switch before. I think I found out by accident from completely wiping the port config and forgetting to put anything back.

This is of course, not a great solution or workaround and trying to search for the problem is kinda difficult given the far more commonplace issues with the same error.

I have seen this happen with Win 7 and some older machines. Possibly a Win 10 but I don’t recall. Cisco Catalyst switches of various models. I’ve removed all but switchport mode access, and switchport access vlan ### from the switchport so it doesn’t seem to depend on anything besides the vlan.

As far as I could tell, not a MAC address issue. After all, then it
would still have a problem even in a different VLAN.

You can have the same MAC in a different VLAN, you cannot have a duplicate MAC on the same VLAN though. The MAC is at L2 and when you connect to a different system in a different VLAN, L2 doesn’t traverse a L3 connection.

If it was a duplicate MAC, then even after changing the VLAN back to the original setting, you would still have the same issue. When the problem does occur, can you take the MAC of that system and ask the L3 gateway for the ARP entry for that MAC? Does the switch that the computer is on, does it show the correct port for the learned MAC address?

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