Zyxel OLT2412 [109/326] Stp terminology

OLT2412 User’s Guide 109

CHAPTER 13

Spanning Tree Protocol

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

to the trees.

13.1 STP/RSTP Overview

(R)STP detects and breaks network loops and provides backup links between switches, bridges or

routers. It allows a OLT 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 OLT 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.

13.1.1 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 52 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