A PC has physical connections to two seperate networks/ subnets. One subnet has internet access, the other is an isolated test network. When the test network is connected the working internet connection fails. What is the issue, how do you know?

A PC has physical connections to two seperate networks/ subnets. One subnet has internet access, the other is an isolated test network. When the test network is connected the working internet connection fails. What is the issue, how do you know?

• There is a broadcast storm on the isolated network, rapid activity lights blinking on connection.
• A PC can't be a member of multiple subnets at once, the internet works when test network is offline
• A route for the isolated network has a metric < or = to internet connection, use route print command
• The PC has an invalid default gateway on the isolated network and can't connect to another network. 

EXPLANATION

2 PC's.
PC1 = 192.168.20.1/24
PC2 = 192.168.30.1/24

PC1 wants to ping PC2
The first thing it does is compare the IP address of PC2 with its own IP/subnetmask. It realises that PC2 is on another network.
PC1 checks its routing table to se if it has a route to PC2's subnet. Most likely it does not. But it should have a default route.
So what PC1 does is do an ARP for the default gateways IP and gets the MAC of the default address. (if it doesnt have a default gateway it then drops the ping)

PC1 then encapsulates the ping (icmp) in a ethernet frame with a destination address = MAC address of default gateway.

So when the ethernet frame arrives at the L2 switch it forwards the frame to the default gateway. NOT PC2.