Specifications

Ethernet Bridge Operation 9-5
Spanning Tree Algorithm
- Message Age: A 2-byte field that contains the age of the
configuration BPDU. This parameter allows a bridge to determine
if the BPDU is too old and needs to be discarded.
- Max Age: A 2-byte field that contains a time-out value initially set
by the root bridge. This value is compared to the Message Age to
determine the validity of the BPDU.
- Hello Time: A 2-byte field that contains the value for the time
interval used to generate configuration BPDU’s by the root bridge.
If a bridge does not hear from the root bridge within the time
defined by the Hello Time, the bridge will initiate a topology
change and attempt to become the new root.
- Forward Delay: A 2-byte field that contains a value used by all
bridges as a delay value when a port changes state to the
forwarding condition. This delay is necessary to prevent data loops
and duplicate data that could be caused by an instantaneous
change of port state. This delay value is also used as the age time for
the source address table whenever a topology change has been
detected by a bridge. This value temporarily replaces the default
age time in order to quickly flush the source address table so
network addresses will be properly relearned after the topology
change. The Forward Delay time value is established by the root
bridge.
The purpose of the configuration BPDU is to notify other bridges on all of
the connected networks of the current topology. Based on the bridge
priority and address, the other bridges automatically detect loops and
negotiate a single path. The bridge or bridges involved in this primary
data path then come on-line and the bridges with lower priority involved
in the backup path(s) remain in standby.
Topology Change BPDU
A topology change BPDU is made up of only 4 data bytes (plus pad) that
contain the Protocol Identifier, Protocol Version Identifier and BPDU Type.
The purpose of the topology change BPDU is to notify other bridges that a
change has taken place. The other bridges then re-span to form a legal
topology.