I’ve got an unusual question a few days ago:
Does a loop (cable returning back to same switch) in one switch affect other switches? How can I detect that there is such a problem in a particular switch?
The correct answer to the first question is obviously it depends. To start with, it depends on whether the two ports will be able to communicate. With a crossover (switch-to-switch) cable (and assuming there are no negotiation issues), the physical layer will work correctly. If you’re using a standard RJ-45 patch cable, you’re “out of luck” unless the switch is too smart and has auto-MDI sensing (like the Linksys switches, now well hidden under obscure part numbers like Cisco SRW248G4). In this case, the two ports will become active even connected with a patch cable.
What happens next depends on how smartly you’ve configured the ports. If you haven’t touched the default configuration, the spanning tree (STP) will discover a loop and disable one of the ports. You can identify a disabled port(s) with the show spanning-tree commands.
If you’ve configured spanning-tree portfast on a port but forgot to configure BDPU guard (or maybe you have an older switch or a low-end router with a switch module that does not support BPDU guard), you’ve just created a nice loop in your bridged network.
I’ve seen interesting network meltdowns following a layer-2 loop. Broadcasts or multicasts are most likely to be looped, so they will affect the whole network, including all connected workstations.