Understanding Bridge Protocol Data Units; Election Of The Root Bridge - Cisco 7604 Configuration Manual
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Understanding How STP Works
Understanding Bridge Protocol Data Units
Bridge protocol data units (BPDUs) are transmitted in one direction from the root bridge. Each network
device sends configuration BPDUs to communicate and compute the spanning tree topology. Each
configuration BPDU contains the following minimal information:
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When a network device transmits a BPDU frame, all network devices connected to the LAN on which
the frame is transmitted receive the BPDU. When a network device 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:
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Election of the Root Bridge
For each VLAN, the network device with the highest-priority bridge ID (the lowest numerical ID value)
is elected as the root bridge. If all network devices are configured with the default priority (32768), the
network device with the lowest MAC address in the VLAN becomes the root bridge. The bridge priority
value occupies the most significant bits of the bridge ID.
When you change the bridge priority value, you change the probability that the router will be elected as
the root bridge. Configuring a higher-priority value increases the probability; a lower-priority value
decreases the probability.
The STP root bridge is the logical center of the spanning tree topology in a Layer 2 network. All paths
that are not needed to reach the root bridge from anywhere in the Layer 2 network are placed in STP
blocking mode.
BPDUs contain information about the transmitting bridge and its ports, including bridge and MAC
addresses, bridge priority, port priority, and path cost. STP uses this information to elect the root bridge
for the Layer 2 network, to elect the root port leading to the root bridge, and to determine the designated
port for each Layer 2 segment.
Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
20-4
The unique bridge ID of the network device that the transmitting network device believes to be the
root bridge
The STP path cost to the root
The bridge ID of the transmitting bridge
Message age
The identifier of the transmitting port
Values for the hello, forward delay, and max-age protocol timers
One network device is elected as the root bridge.
The shortest distance to the root bridge is calculated for each network device based on the path cost.
A designated bridge for each LAN segment is selected. This is the network device 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.
Chapter 20
Configuring STP and Prestandard IEEE 802.1s MST
OL-4266-08
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