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Cisco IE 2000 Switch Software Configuration Guide
OL-25866-01
Chapter 20 Configuring STP
Information About Configuring STP
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:
• 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
When a switch receives a configuration BPDU that contains superior information (lower bridge ID,
lower path cost, and so forth), it stores the information for that port. If this BPDU is received on the root
port of the switch, the switch also forwards it with an updated message to all attached LANs for which
it is the designated switch.
If a switch receives a configuration BPDU that contains inferior information to that currently stored for
that port, it discards the BPDU. If the switch is a designated switch for the LAN from which the inferior
BPDU was received, it sends that LAN a BPDU containing the up-to-date information stored for that
port. In this way, inferior information is discarded, and superior information is propagated on the
network.
A BPDU exchange results in these actions:
• One switch in the network is elected as the root switch (the logical center of the spanning-tree
topology in a switched network).
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 switch priority value
occupies the most significant bits of the bridge ID, as shown in Table 20-1 on page 20-4.
• A root port is selected for each switch (except the root switch). This port provides the best path
(lowest cost) when the switch forwards packets to the root switch.
• The shortest distance to the root switch is calculated for each switch based on the path cost.
• A designated switch for each LAN segment is selected. The designated switch incurs the lowest path
cost when forwarding packets from that LAN to the root sw itch. The port through which the
designated switch is attached to the LAN is called the designated port.
All paths that are not needed to reach the root switch from anywhere in the switched network are placed
in the spanning-tree blocking mode.
Bridge ID, Switch Priority, and Extended System ID
The IEEE 802.1D standard requires that each switch has an u nique bridge identifier (bridge ID), which
controls the selection of the root switch. Because each VLAN is considered as a different logical bridge
with PVST+ and rapid PVST+, the same switch must have a different bridge IDs for each configured
VLAN. Each VLAN on the switch has a unique 8-byte bridge ID. The 2 most- significant bytes are used
for the switch priority, and the remaining 6 bytes are derived from the switch MAC address.