OmniSwitch os6900, Руководство по настройке сети

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Spanning Tree Overview Configuring Spanning Tree Parameters
page 6-6 OmniSwitch AOS Release 7 Network Configuration Guide June 2013
Spanning Tree Overview
Alcatel-Lucent switches support 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 an
Alcatel-Lucent 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
“MST General Overview” on page 6-12 for specific
information about 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.