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Catalyst2950 and Catalyst2955 Switch Software Configuration Guide
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Chapter14 Configuring STP Understanding Spanning-Tree Features
Spanning-Tree Topology and BPDUs
The stable, active spanning-tree topology of a switched network is determined by these elements:
•The unique bridge ID (switch priority and MAC address) associated with each VLAN on eac h switch
•The spanning-tree path cost to the root switch
•The port identifier (port priority and MAC address) associated with each Layer 2 interface
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 roo t
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 logi ca l c enter of the sp an ning- 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 Table14-1 on page14-4.
•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 d esignated swi tch incur s the lo west path
cost when forwarding packets from that LAN to the root sw itch . Th e por t thr ough whi ch th e
designated switch is attached to the LAN is called the designated port.
•Interfaces included in the spanning-tree instance are selected. Root ports and designated ports are
put in the forwarding state.
•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.