Quiz Entry - updated: 2026.07.14
What is the internal root path cost, and what are the default port costs for common link speeds?
The internal root path cost is the cumulative cost of all ports along the path from a switch to the root bridge.
When a switch receives a BPDU (Bridge Protocol Data Unit), it adds the ingress port cost to the received root path cost. This determines the total cost to reach the root bridge via that path.
Default port costs (IEEE 802.1D short path cost):
| Link Speed | STP (Spanning Tree Protocol) Cost (802.1D-1998) | RSTP (Rapid Spanning Tree Protocol) Cost (802.1w-2004) |
|---|---|---|
| 10 Gbps | 2 | 2,000 |
| 1 Gbps | 4 | 20,000 |
| 100 Mbps | 19 | 200,000 |
| 10 Mbps | 100 | 2,000,000 |
Key points:
- Cisco switches use the short path cost (802.1D values) by default for both STP and RSTP
- Faster links = lower cost (preferred paths)
- Port costs are configurable — administrators can manually influence which paths STP prefers
- RSTP uses larger numbers to provide better granularity between modern high-speed links
Tip: The cost is inversely proportional to bandwidth. A 1 Gbps link (cost 4) is preferred over a 100 Mbps link (cost 19).
Go deeper:
Spanning Tree Protocol — path cost (Wikipedia) — the per-speed cost table (short vs long values) and the cumulative-cost rule.