Fixed Switch Configuration Guide 15-1
15
Configuring Spanning Tree
This chapter provides the following information about configuring and monitoring the Spanning
Tree protocol on Enterasys stackable and standalone fixed switches.
Spanning Tree Protocol Overview
The Spanning Tree Protocol (STP) resolves the problem of physical loops in a network by
establishing one primary path between any two devices. Duplicate paths are barred from use and
become standby or “blocked” paths until the primary path fails, at which point the redundant
path can be brought into service.
STP operates by forming a fully connected tree of data loop free LAN connected bridges
(switches) through the exchange of Bridge Protocol Data Units (BPDUs). Each bridge port
transmits BPDUs on a periodic basis. The information contained in the BPDU is used by the
receiving bridge to calculate a port role for each bridge port. There is one bridge in the network
chosen to be the root bridge, based on its bridge ID. Ports that directly connect bridges to the root
bridge or are connected through another bridge are assigned one of four roles:
• Root Port – The best path to the root
• Designated Port – Ports which either provide a path to the root for other bridges or connect
end users
• Backup Port – A port attached to a LAN where another port of the same bridge is a designated
port. This backup port takes over the designated role should the LAN’s designated port
become disabled
• Alternate Port – A port providing a path to the root that is not root, designated, or backup
For a summary of port roles, see Table 15-2 on page 15-13.
For information about... Refer to page...
Spanning Tree Protocol Overview 15-1
STP Operation 15-3
Functions and Features Supported on Enterasys Devices 15-6
Spanning Tree Basics 15-9
Configuring STP and RSTP 15-19
Configuring MSTP 15-24
Understanding and Configuring SpanGuard 15-29
Understanding and Configuring Loop Protect 15-31
Terms and Definitions 15-36
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