Determining the Designated Port on Each LAN Segment
Each LAN segment has a single switch that acts as the designated port (DP) on that segment. On segments that connect a switch to a device that does not even use STP—for example, segments connecting a switch to a PC or a router—the switch always wins, because it is the only device sending a hello onto the link. However, links with two switches require a little more work to discover which should be the DP. By definition:
Step 1. For switches connected to the same LAN segment, the switch with the lowest cost to reach the root, as advertised in the hello they send onto the link, becomes the DP on ...
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