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Software Configuration Guide—Release 12.2(25)SG
OL-7659-03
Chapter13 Understanding and Conf iguring STP
Overview of STP

STP MAC Address Allocation

A Catalyst 4500 series switch chassis has either 64 or 1024 MAC addresses available to support software
features like STP. Enter the show module command to view the MAC address range on your chassis.
Release 12.1(12c)EW and later releases support chassis with 64 or 1024 MAC addresses. For chassis
with 64 MAC addresses, STP uses the extended system ID plus a MAC address to make the brid ge ID
unique for each VLAN.
Earlier releases support chassis with 1024 MAC addresses. With earlier releases, STP uses one MAC
address per VLAN to make the bridge ID unique for each VLAN.
Bridge Protocol Data Units
The following elements determine the stable active spanning tr ee topology of a switched network:
•The unique bridge ID (bridge priority and MAC address) associated with ea ch VLAN on each switch
•The spanning tree path cost (or bridge priority value) to the root bridge
•The port identifier (port priority and MAC address) associated with each Layer 2 interface
Bridge protocol data units (BPDUs) contain information about the transmitting bridge and its ports,
including the bridge and MAC addresses, bridge priority, port priority, and path cost. The system
computes the spanning tree topology by transmitting BPDUs among connecting switches, and in one
direction from the root switch. Each configuration BPDU contains at least the following:
•The unique bridge ID of the switch that the transmitting switch believes to be the root switch
•The spanning tree path cost to the root
•The bridge ID of the transmitting bridge
•The age of the message
•The identifier of the transmitting port
•Values for the hello, forward delay, and max-age protocol timers
When a switch transmits a BPDU frame, all switches connected to the LAN on which the frame is
transmitted 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, initiate a BPDU
transmission.
A BPDU exchange results in the following:
•One switch is elected as the root bridge.
•The shortest distance to the root bridge is calculated for each switch based on the path cost.
•A designated bridge for each LAN segment is selected. This is the switch closest to the root bridge
through which frames are forwarded to the root.
•A root port is selected. This is the port providing the best path from the bridge to the root bridge.
•Ports included in the spanning tree are selected.