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7 Spanning Tree Algorithm
This chapter describes the following basic topics:
◆ Loopback Detection – Configures detection and response to loopback BPDUs.
◆ Global Settings for STA – Configures global bridge settings for STP, RSTP and
MSTP.
◆ Interface Settings for STA – Configures interface settings for STA, including
priority, path cost, link type, and designation as an edge port.
◆ Global Settings for MSTP – Sets the VLANs and associated priority assigned to
an MST instance
◆ Interface Settings for MSTP – Configures interface settings for MSTP, including
priority and path cost.
Overview
The Spanning Tree Algorithm (STA) can be used to detect and disable network
loops, and to provide backup links between switches, bridges or routers. This
allows the switch to interact with other bridging devices (that is, an STA-compliant
switch, bridge or router) in your network to ensure that only one route exists
between any two stations on the network, and provide backup links which
automatically take over when a primary link goes down.
The spanning tree algorithms supported by this switch include these versions:
◆ STP – Spanning Tree Protocol (IEEE 802.1D)
◆ RSTP – Rapid Spanning Tree Protocol (IEEE 802.1w)
◆ MSTP – Multiple Spanning Tree Protocol (IEEE 802.1s)
STP – STP uses a distributed algorithm to select a bridging device (STP-compliant
switch, bridge or router) that serves as the root of the spanning tree network. It
selects a root port on each bridging device (except for the root device) which incurs
the lowest path cost when forwarding a packet from that device to the root device.
Then it selects a designated bridging device from each LAN which incurs the lowest
path cost when forwarding a packet from that LAN to the root device. All ports
connected to designated bridging devices are assigned as designated ports. After
determining the lowest cost spanning tree, it enables all root ports and designated
ports, and disables all other ports. Network packets are therefore only forwarded
between root ports and designated ports, eliminating any possible network loops.
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