Understanding Bridge Protocol Data Units; Election Of The Root Bridge - Cisco 7609 Configuration Manual
Cisco ios software configuration guide—12.1e
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Understanding How STP Works
If you have a network device in your network with MAC address reduction enabled, you should also
enable MAC address reduction on all other Layer-2 connected network devices to avoid undesirable root
bridge election and spanning tree topology issues.
When MAC address reduction is enabled, the root bridge priority becomes a multiple of 4096 plus the
VLAN ID. With MAC address reduction enabled, a switch bridge ID (used by the spanning-tree
algorithm to determine the identity of the root bridge, the lowest being preferred) can only be specified
as a multiple of 4096. Only the following values are possible: 0, 4096, 8192, 12288, 16384, 20480,
24576, 28672, 32768, 36864, 40960, 45056, 49152, 53248, 57344, and 61440.
If another bridge in the same spanning-tree domain does not run the MAC address reduction feature, it
could win root bridge ownership because of the finer granularity in the selection of its bridge ID.
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 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 value increases the probability; a lower value decreases the
probability.
Cisco 7600 Series Router Cisco IOS Software Configuration Guide—12.1E
15-4
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 15
Configuring STP and IEEE 802.1s MST
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