Spanning Tree Overview; How The Spanning Tree Topology Is Calculated - Alcatel OmniSwitch 6624 Network Configuration Manual

Configuring Spanning Tree Parameters

Spanning Tree Overview

Alcatel switches support the use of the traditional STP defined in the IEEE 802.1D standard and the Rapid
Spanning Tree Algorithm and Protocol (RSTP) defined in the IEEE 802.1w standard. RSTP expedites
topology changes by allowing blocked ports to transition directly into a forwarding state, bypassing listen-
ing and learning states. This provides rapid reconfiguration of the Spanning Tree in the event of a network
path or device failure.
The 802.1w standard is an amendment to the 802.1D document, thus RSTP is based on STP. Although
802.1D is supported, the 802.1w document recommends implementing RSTP whenever possible. Regard-
less of which protocol a switch or VLAN is running, it can successfully interoperate with other switches or
VLANs. This section provides a Spanning Tree overview based on RSTP operation and terminology.

How the Spanning Tree Topology is Calculated

The tree consists of links and bridges that provide a single data path that spans the bridged network. At the
base of the tree is a root bridge. One bridge is elected by all the bridges participating in the network to
serve as the root of the tree. After the root bridge is identified, STP calculates the best path that leads from
each bridge back to the root and blocks any connections that would cause a network loop.
To determine the best path to the root, STP uses the path cost value, which is associated with every port
on each bridge in the network. This value is a configurable weighted measure that indicates the contribu-
tion of the port connection to the entire path leading from the bridge to the root. The IEEE 802.1D stan-
dard recommends using the following path cost values based on link speed:
Link Speed
4 Mbps
10 Mbps
16 Mbps
100 Mbps
1 Gbps
10 Gbps
In addition, a root path cost value is associated with every bridge. This value is the sum of the path costs
for the port that receives frames on the best path to the root (this value is zero for the root bridge). The
bridge with the lowest root path cost becomes the designated bridge for the LAN, as it provides the short-
est path to the root for all bridges connected to the LAN.
During the process of calculating the Spanning Tree topology, each port on every bridge is assigned a port
role based on how the port and/or its bridge will participate in the active Spanning Tree topology. The
following table provides a list of port role types and the port and/or bridge properties that the Spanning
Tree Algorithm examines to determine which role to assign to the port.
OmniSwitch 6624/6648 Network Configuration Guide
Recommended Recommended
Value
250 100–1000
100
62
19
4
2
Range
Range
1–65535
50–600 1–65535
40–400 1–65535
10–60 1–65535
3–10 1–65535
1–5 1–65535
April 2004
Spanning Tree Overview
page 5-3