The topology
Four switches, all with the default priority 32768. They run classic STP with short path costs. Notice that one link is slower than the others.
- 1. Your job: work out the tree these switches will build. Write down each answer before you open it.
| Link | Speed | STP cost | Ports |
|---|---|---|---|
| SW-A – SW-B | 1 Gbps | 4 | A Gi0/1 ↔ B Gi0/1 |
| SW-A – SW-C | 100 Mbps | 19 | A Gi0/2 ↔ C Gi0/1 |
| SW-B – SW-D | 1 Gbps | 4 | B Gi0/2 ↔ D Gi0/1 |
| SW-C – SW-D | 1 Gbps | 4 | C Gi0/2 ↔ D Gi0/2 |
Task 1: which switch is the root bridge?
Show answer
SW-A. All priorities are 32768, so the lowest MAC address wins: …000A.
Task 2: the root port and cost of each other switch
💡 Start at the root and work outward: B first, then D, then C.
Show answer
| Switch | Options | Root port | Cost |
|---|---|---|---|
| SW-B | Direct to A: 4 | Gi0/1 | 4 |
| SW-D | Through B: 4 + 4 = 8 · through C: 4 + C's cost | Gi0/1 (toward B) | 8 |
| SW-C | Direct to A: 19 · through D: 4 + 8 = 12 | Gi0/2 (toward D) | 12 |
The surprise is SW-C. Its direct link to the root costs 19. But the long way round, through D and B, costs only 12. STP compares cost, not the number of switches.
Task 3: the designated port on each link
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| Link | Compare root path cost | Designated port |
|---|---|---|
| A – B | A 0 vs B 4 | SW-A Gi0/1 |
| A – C | A 0 vs C 12 | SW-A Gi0/2 |
| B – D | B 4 vs D 8 | SW-B Gi0/2 |
| C – D | D 8 vs C 12 | SW-D Gi0/2 |
Task 4: which port is blocked?
Show answer and the finished tree
SW-C Gi0/1, its port on the slow link to SW-A. It is not C's root port, and SW-A's end of the link is designated. So this port is blocked.
- 1. SW-C's path to the root: C → D → B → A, total cost 12. It avoids the slow direct link.
- 2. The slow link is the backup. SW-C Gi0/1 is blocked. It only forwards if the path through D fails.
Bonus: upgrade the slow link
The A–C link is upgraded to 1 Gbps (cost 4). Work everything out again.
Show answer
- SW-C: direct to A = 4, so its root port becomes Gi0/1 (cost 4).
- SW-D: through B = 8, through C = 8. It's a tie. The lower sender bridge ID wins, which is SW-B (…000B). So D's root port stays Gi0/1.
- Link C–D: C is cost 4, D is cost 8, so SW-C Gi0/2 is designated.
- Blocked: SW-D Gi0/2 (toward C). The blocked port has moved to the other side of the square.