Local Network Loses Internet When Certain Device Connects

Solution 1:

It may be possible that some device has assumed a wrong IP address, so that you might have two devices with the same IP. This might cause problems, especially if that IP address belongs to a switch or some gateway.

You could disconnect all the devices on switch #3, to see if the problem still arrives.

If the problem has disappeared, reconnect them one-by-one to find the malfunctioning device.

Solution 2:

This sounds very much like a routing loop, ie switch 3 has more then 1 path to the router - maybe a direct connection and a connection vua another switch?

You can diagnose this problem by tracing the connections from switch3 back to the other end. A lazier way, which might not work as well would be to discomnect everything from switch 3 and gradually re-add devices until it crashes.

Solution 3:

It's difficult to diagnose without more details, but beyond what is described in the other answers:

• Did you make any changes to your network setup? Added or removed devices, changed configuration, performed an upgrade?

• Do the devices still reach each other? I.e. is it just access to the Internet which stops working, or all network communication? Of course you may need to performs tests between many different pairs of devices to isolate the issue.

• Some problems can show up quite late after a change. For instance, if there's an issue with DHCP, you may not notice it until the current DHCP leases expire (which can be quite long).

• In addition to having multiple devices configured with the same IP, another potential issue is multiple devices acting as DHCP servers.

• It's not unheard of to have multiple devices with the same MAC address! Shouldn't happen, but...

• Loops in the network are normally detected and stopped by the spanning-tree protocol (STP / RSTP), but depending on the exact setup...

• You may have a faulty or infected device that is flooding the network with traffic. If that traffic is itself amplified by other devices it can DoS your network.

As others have written, you should continue your diagnosis until you find a single device which causes the disruption. Depending on the type of device, there can be various ways to find the exact cause and then fix the problem.

Solution 4:

Last time I encountered an issue like this, it was due to the addition of an access point configured for 'routing/DHCP' to the network. Having 2 routers set for DHCP took everything offline. This would be an extension of the 'IP conflict" issue proposed in a previous answer. Connecting to the router and seeing what errors are being tossed back is the best bet IMO to get an accurate diagnosis.

Once on the network, you can use This documentation to get into the router.