Zyxel XS3900-48F [107/332] Stp rstp overview

XS3900-48F User’s Guide 107

CHAPTER 11

Spanning Tree Protocol

The Switch supports Spanning Tree Protocol (STP), Rapid Spanning Tree Protocol (RSTP) and

Multiple Spanning Tree Protocol (MSTP) as defined in the following standards.

• IEEE 802.1D Spanning Tree Protocol

• IEEE 802.1w Rapid Spanning Tree Protocol

• IEEE 802.1s Multiple Spanning Tree Protocol

The Switch also allows you to set up multiple STP configurations (or trees). Ports can then be

assigned to the trees.

11.1 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-compliant switches in your network to

ensure that only one path exists between any two stations on the network.

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 and then the root bridge 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.

Note: In this user’s guide, “STP” refers to both STP and RSTP.

11.1.1 STP Terminology

The root bridge is the base of the spanning tree.

Path cost is the cost of transmitting a frame onto a LAN through that port. The recommended cost

is assigned according to the speed of the link to which a port is attached. The slower the media, the

higher the cost.

Table 31 STP Path Costs

LINK SPEED

RECOMMENDED

VALUE

RECOMMENDED

RANGE

ALLOWED RANGE

Path Cost 4Mbps 250 100 to 1000 1 to 65535

Path Cost 10Mbps 100 50 to 600 1 to 65535

Path Cost 16Mbps 62 40 to 400 1 to 65535

Path Cost 100Mbps 19 10 to 60 1 to 65535