Spanning Tree Overview; How The Spanning Tree Topology Is Calculated - Alcatel-Lucent OmniSwitch 9900 Series Network Configuration Manual

Configuring Spanning Tree Parameters

Spanning Tree Overview

The OmniSwitch supports the use of the 802.1D Spanning Tree Algorithm and Protocol (STP), the 802.1w
Rapid Spanning Tree Algorithm and Protocol (RSTP), and the 802.1Q 2005 Multiple Spanning Tree
Protocol (MSTP).
RSTP expedites topology changes by allowing blocked ports to transition directly into a forwarding state,
bypassing listening 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. Regardless
of which one of these two protocols a switch or VLAN is running, it can successfully interoperate with
other switches or VLANs.
802.1Q 2005 is a new version of MSTP that combines the 802.1D 2004 and 802.1S protocols. This
implementation of 802.1Q 2005 also includes improvements to edge port configuration and provides
administrative control to restrict port role assignment and the propagation of topology change information
through bridge ports.
MSTP is an enhancement to the 802.1Q Common Spanning Tree (CST), which is provided when a switch
is running in the flat Spanning Tree operating mode. The flat mode applies a single spanning tree instance
across all VLAN port connections on a switch. MSTP allows the configuration of Multiple Spanning Tree
Instances (MSTIs) in addition to the CST instance. Each MSTI is mapped to a set of VLANs. As a result,
the flat mode can now support the forwarding of VLAN traffic over separate data paths.
This section provides a Spanning Tree overview based on RSTP operation and terminology. Although
MSTP is based on RSTP, see
configuring MSTP.

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
contribution of the port connection to the entire path leading from the bridge to the root.
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
shortest 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 participates 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 AOS Release 8 Network Configuration Guide
"MST General Overview" on page 6-12
December 2017
Spanning Tree Overview
for specific information about
page 6-5