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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 10 Configuring STP
Understanding Basic STP Features
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The removal of loops in the switched network by blocking Layer 2 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 switch priority value occupies the
most significant bits of the bridge ID.
When you change the switch priority value, you change the probability that the switch will be elected as
the root switch. Configuring a higher value increases the probability; a lower value decreases the
probability.
The 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 sending 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 and 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:
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The unique bridge ID (switch priority and MAC address) associated with each VLAN on each
switch
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The STP path cost to the root switch
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The port identifier (port priority and MAC address) associated with each Layer 2 interface
The BPDUs are sent 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:
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The unique bridge ID of the switch that the sending switch identifies as the root switch
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The STP path cost to the root
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The bridge ID of the sending switch
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Message age
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The identifier of the sending interface
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Values for the hello, forward delay, and max-age protocol timers
When a switch sends a BPDU frame, all switches connected to the LAN on which the frame is sent
receive the BPDU. When a switch receives a BPDU, it does not forward the frame but instead uses the
information in the frame to calculate a BPDU, and, if the topology changes, starts a BPDU transmission.
A BPDU exchange results in these actions:
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One switch is elected as the root switch.
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The shortest distance to the root switch is calculated for each switch based on the path cost.
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A designated switch for each LAN segment is selected. The designated switch is the one closest to
the root switch through which frames are forwarded to the root.
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A root port is selected. This port provides the best path from the switch to the root switch.
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Interfaces included in the spanning-tree instance are selected.
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All interfaces not included in the spanning tree are blocked.
