Configuring Stp; Understanding Spanning-Tree Features; Stp Overview; Supported Spanning-Tree Instances - Cisco Catalyst 2950 Software Configuration Manual

Understanding Spanning-Tree Features

STP Overview

STP is a Layer 2 link management protocol that provides path redundancy while preventing loops in the
network. For a Layer 2 Ethernet network to function properly, only one active path can exist between
any two stations. Spanning-tree operation is transparent to end stations, which cannot detect whether
they are connected to a single LAN segment or a switched LAN of multiple segments.
When you create fault-tolerant internetworks, you must have a loop-free path between all nodes in a
network. The spanning-tree algorithm calculates the best loop-free path throughout a switched Layer 2
network. Switches send and receive spanning-tree frames, called bridge protocol data units (BPDUs), at
regular intervals. The switches do not forward these frames, but use the frames to construct a loop-free
path.
Multiple active paths among end stations cause loops in the network. If a loop exists in the network, end
stations might receive duplicate messages. Switches might also learn end-station MAC addresses on
multiple Layer 2 interfaces. These conditions result in an unstable network.
Spanning tree defines a tree with a root switch and a loop-free path from the root to all switches in the
Layer 2 network. Spanning tree forces redundant data paths into a standby (blocked) state. If a network
segment in the spanning tree fails and a redundant path exists, the spanning-tree algorithm recalculates
the spanning-tree topology and activates the standby path.
When two interfaces on a switch are part of a loop, the spanning-tree port priority and path cost settings
determine which interface is put in the forwarding state and which is put in the blocking state. The port
priority value represents the location of an interface in the network topology and how well it is located
to pass traffic. The path cost value represents media speed.

Supported Spanning-Tree Instances

The switch supports the per-VLAN spanning tree (PVST) and a maximum of 64 spanning-tree instances.
For information about how spanning tree interoperates with the VLAN Trunking Protocol (VTP), see the
"STP Configuration Guidelines" section on page

Bridge Protocol Data Units

The stable, active spanning-tree topology of a switched network is determined by these elements:
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When the switches in a network are powered up, each functions as the root switch. Each switch sends a
configuration BPDU through all of its ports. The BPDUs communicate and compute the spanning-tree
topology. Each configuration BPDU contains this information:
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Catalyst 2950 Desktop Switch Software Configuration Guide
10-2
The unique bridge ID (switch priority and MAC address) associated with each VLAN on each switch
The spanning-tree path cost to the root switch
The port identifier (port priority and MAC address) associated with each Layer 2 interface
The unique bridge ID of the switch that the sending switch identifies as the root switch
The spanning-tree path cost to the root
The bridge ID of the sending switch
Message age
The identifier of the sending interface
Values for the hello, forward delay, and max-age protocol timers
10-10.
Chapter 10

Configuring STP

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