Spanning Tree Protocol Designated Ports

You may have noticed that we haven’t talked about loops yet. If you look back at Figure 3-5, a loop definitively exists. On each network segment, one port needs to be chosen at the Designated Port. The responsibility of the Designated Port is to act as the single interface to forward traffic destined for the Root Bridge. Recall that in our network example, 3 segments exist. Refer back to Figure 5 to review the costs associated with each port on our network.

To choose the Designated Port, another election needs to take place. Bridges compare their port costs to decide who gets to be the Designated Port for that segment. Consider each segment in this figure:

On segment AC, the Designated Port will be port 2 on bridge A. That’s because port 2 on bridge A has a cost of 0, while port 1 on bridge C has a cost of 19.

On segment AB, the Designated Port will be port 1 on bridge A. Again, port 1 on bridge A has a cost of 0, while port 1 on bridge B has a cost of 19. Since their port cost is always 0, it should be clear that ports on the Root Bridge will always be Designated Ports for their connected segments.

On segment BC, there is a tie. Port 2 on each bridge has a cost of 19. As such, the Designated Port will be the switch with the lowest MAC address. In this case that’s bridge B, so port 2 on bridge B will become the Designated Port.

Note that after all this is done, all traffic from segment BC will be forwarded out port 2 on switch B. Port 2 on switch C will be put into blocking mode, as shown in the figure below. Notice also that there are no longer any loops on our network.

Figure: Designated Ports for each segment.

On any given segment, the bridge that has the current Designated Port is also known as the designated bridge.

Author: Dan DiNicolo

Dan DiNicolo is a freelance author, consultant, trainer, and the managing editor of He is the author of the CCNA Study Guide found on this site, as well as many books including the PC Magazine titles Windows XP Security Solutions and Windows Vista Security Solutions. Click here to contact Dan.