Effect of a Network Loop
So what happens when there is a loop on your Cisco network and you do not have spanning tree running? Well, if you have no network traffic on your network, then nothing. If you have a hub-based network, as soon as the first piece of data is sent on the network, a single Ethernet frame will cycle through the loop repeatedly.
A single network frame will cycle around the network and actually consume 100% of the possible network bandwidth.
If you have a switch-based network, then it may actually take a broadcast packet to cause a problem with a network loop. When a switch gets a broadcast from a network device, it forwards it out through all ports. The neighboring switch will get that broadcast and forward the broadcast through all other ports, and due to the loop, this broadcast will make its way to the original switch that received the broadcast from the network device.
When the broadcast arrives, it will not know that it has seen it before, so it will forward it to all other ports. This process will be repeated thousands of times per second, causing a huge volume of traffic from a single broadcasted Ethernet frame.
When this happens on your network, everyone will lose the ability to communicate on the network, and the activity lights on your switches will be solid (on) rather than blinking (on and off). If you break the loop, your network will return to normal in a few minutes.