recently we had a lecture in network engineering about the STP/RSTP and learned the basics, but a friend of mine and me asked us what would happen if one would make the mistake to connect Port 1 and Port2 of a Bridge - which Roles would the Ports have? I tried to find some information on the topic but i couldn't end up finding something that was related to the question. 2 ports of a Bridge connected

the Cisco docs on STP Porrt Roles say:

Backup—This is a special case when two or more ports of the same bridge (switch) are connected together, directly or through shared media. In this case, one port is designated, and the remaining ports block. The role for this port is backup. from Cisco STP Doc

I am not 100% sure which port does what - is it lowest port number first or am i misunderstanding this explanation?

Thanks for your input!

  • 1
    See What happens if... May 23, 2014 at 14:30
  • 2
    I would mention that this scenario is different than the one Craig linked because RSTP is running here, but the linked question only uses consumer switches with no spanning tree. RSTP should manage this loop May 23, 2014 at 16:05

1 Answer 1


You are on the right track. The port with the lowest port ID (port 1) will become the designated port, and the other port will be an alternate, and therefore will be discarding.

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