10
Spanning Tree Protocol
This chapter introduces the Spanning Tree Protocol (STP) and Rapid Spanning Tree Protocol (RSTP).
10.1 STP/RSTP Overview
(R)STP detects and breaks network loops and provides backup links between switches, bridges or routers. It allows a switch to interact with other (R)STP
The switch uses IEEE 802.1w RSTP (Rapid Spanning Tree Protocol) that allows faster convergence of the spanning tree than STP (while also being backwards compatible with STP- only aware bridges). In RSTP, topology change information is directly propagated throughout the network from the device that generates the topology change. In STP, a longer delay is required as the device that causes a topology change first notifies the root bridge that then notifies the network. Both RSTP and STP flush unwanted learned addresses from the filtering database. In RSTP, the port states are Discarding, Learning, and Forwarding.
"In this user’s guide, “STP” refers to both STP and RSTP.
10.1.1STP Terminology
The root bridge is the base of the spanning tree; it is the bridge with the lowest identifier value (MAC address).
Path cost is the cost of transmitting a frame onto a LAN through that port. It is assigned according to the speed of the link to which a port is attached. The slower the media, the higher the cost.
Table 24 STP Path Costs
| LINK | RECOMMENDED | RECOMMENDED | ALLOWED |
| SPEED | VALUE | RANGE | RANGE |
Path | 4Mbps | 250 | 100 to 1000 | 1 to 65535 |
Cost |
|
|
|
|
|
|
|
|
|
Path | 10Mbps | 100 | 50 to 600 | 1 to 65535 |
Cost |
|
|
|
|
|
|
|
|
|
Path | 16Mbps | 62 | 40 to 400 | 1 to 65535 |
Cost |
|
|
|
|
|
|
|
|
|
| 101 |
|
|