Find the Root Bridge

1. Find the root bridge with the lowest Bridge ID. (Bridge priority and MAC address)
2. All the active ports on the Root Bridge become designated ports.




Find the Root ports (One RP per switch)

1. Lowest cost to Root Bridge.
2. Lowest neighbor Bridge ID

3. Lowest neighbor port priority
4. Lowest neighbor internal port number.

Note: 3-4 are only used if you have multiple links between two switches and if you tie with 1-2.





Finding the Designated ports for LAN segments.

1. Find the switch that has the lowest cost to the root bridge.
You do this by looking at the link between the switches. Which side has the lowest cost? Then that side of the link becomes the DP.
2. Which side of the link has the lowest BID

3. Lowest port priority
4. Lowest internal port number.

Note: 3-4 are super rare for DPs. It's only if you have multiple links connected between two switches through a HUB! In this situation, you created a loop! This never happens in real networks.




After you find the RPs and DPs all other ports become blocked.



STP Costs
1. 100 for 10Mbps
2. 19 for 100Mbps
3. 4 for 1Gbps
4. 2 for 10Gbps


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