How Spanning Tree Protocol (STP) select Designated Port
Spanning Tree Protocol (STP) must elect a Designated Port on a switch for each network segment. Following are the different steps for selecting the Designated Port.
• First step is to select the switch on the segment. The switch on the network segment with the lowest accumulated path cost to the Root Bridge will be selected.
• If there is a tie in accumulated path costs between the two switches in the network segment, then the switch with the lowest Switch ID will be selected.
• If a switch has separate connections to the network segment, the switch port with the lowest port priority (32 by default) is selected.
• If there is again a tie (it can happen if the priorities of the ports on this switch are the same), then the lowest numbered port on the switch is selected.
