> For the complete documentation index, see [llms.txt](https://davidjosearaujo.gitbook.io/online-courses/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://davidjosearaujo.gitbook.io/online-courses/ccna-200-301/spanning-tree-protocol/stp-port-role-selection.md).
# 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.
