STP Port Role Selection

1. The switch with the lowest bridge ID is elected as the root bridge. All ports on the root bridge are designated ports (forwarding state). Root bridge selection:

   1. Lowest bridge ID.

2. Each remaining switch will select ONE of its interfaces to be its root port. The interface with the lowest root cost will be the root port. Root bridge selection:

   1. Lowest root cost.

   2. Lowest neighbor bridge ID.

   3. Lowest neighbor port ID.

3. Each remaining collision domain will select one interface to be a designated port (forwarding state). The other port in the collision will be non-designated (bocking). Designated port selection:

   1. The interface on the switch with the lowest root cost.

   2. The interface on the switch with the lowest bridge ID.

Speed	STP Cost
10 Mbps	100
100 Mbps	19
1 Gbps	4
10 Gbps	2

SW3's logic

• I was advertised a cost of 0 on G0/0. My interface cost = 4. Total cost via G0/0 = 4.

• I was advertised a cost of 4 on G0/1. My interface cost = 4. Total cost via G0/1 = 8.

The ports connected to another switch's root port must be designated. Because the root port is the switch's path to the root bridge, another switch must not block it.

Port ID

STP Port ID = port priority (default 128) + port number.