9-3
Catalyst 2950 Desktop Switch Software Configuration Guide
78-11380-03
Chapter 9 Configuring STP
Understanding Basic STP Features
Multiple active paths between 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 interfaces. These conditions result in an unstable network.
STP defines a tree with a root switch and a loop-free path from the root to all switches in the network.
STP 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 STP port priority and path cost settings determine
which interface is put in the forwarding state and which is put in the blocking state. The STP port priority
value represents the location of an interface in the network topology and how well it is located to pass
traffic. The STP path cost value represents media speed.
Election of the Root Switch
All switches in the network participating in STP gather information about other switches in the network
through an exchange of data messages called Bridge Protocol Data Units (BPDUs). This exchange of
messages results in these actions:
• The election of a unique root switch for each instance of spanning tree
• The election of a designated switch for every switched LAN segment
• The removal of loops in the switched network by blocking interfaces connected to redundant links
For each VLAN, the switch with the highest switch priority (the lowest numerical priority value) is
elected as the root switch. If all switches are configured with the default priority (32768), the switch with
the lowest MAC address in the VLAN becomes the root switch.
The spanning-tree root switch is the logical center of the STP topology in a switched network. All paths
that are not needed to reach the root switch from anywhere in the switched network are placed in STP
blocking mode.
BPDUs contain information about the transmitting switch and its ports, including switch and MAC
addresses, switch priority, port priority, and path cost. STP uses this information to elect the root switch
and root port for the switched network, as well as the root port and designated port for each switched
segment.
Bridge Protocol Data Units
The stable, active STP topology of a switched network is determined by these elements:
• The unique bridge ID (switch priority and MAC address) associated with each VLAN on each switch
• The STP path cost to the root switch
• The port identifier (port priority and MAC address) associated with each interface
The BPDUs are transmitted in one direction from the root switch, and each switch sends configuration
BPDUs to communicate and to compute the STP topology. Each configuration BPDU contains this
information:
• The unique bridge ID of the switch that the transmitting switch identifies as the root switch
• The STP path cost to the root
• The bridge ID of the transmitting switch
• Message age
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