Specifications

Spanning Tree Algorithm
9-4 Ethernet Bridge Operation
Configuration BPDU
When a bridge is powered up, it goes through a series of self tests to check
its internal operation. During this time the bridge is in a standby condition
and does not forward traffic. Also during this standby period, the bridge
sends out special bridge management frames called Configuration Bridge
Protocol Data Units (BPDU). Bridges use the BPDU frames as a way of
communicating with each other. The configuration BPDU is 64 bytes long
and contains the following fields:
• Destination Address: A specific 6-byte Ethernet multicast address that
denotes the bridge group.
• Source Address: The standard 6-byte Ethernet hardware address of the
bridge transmitting the BPDU.
• Length Field: A standard, 2-byte, IEEE 802.3 data field length.
• Data Field: Contains 35 bytes for BPDU data and 13 bytes of pad to
equal the minimum data field size of 48 bytes. The BPDU data field
contains the following information:
- Protocol Identifier: A reserved, 2-byte, protocol identifier defined
by IEEE.
- Protocol Version Identifier: Identifies the version of the bridge
protocol being used and is 1 byte in length.
- BPDU Type: 1 byte to identify the BPDU type as either a
configuration or topology change BPDU.
- Flags: A 1-byte field that contains topology change and topology
change acknowledgment flags.
- Root Identifier: An 8-byte identification number derived from the
Ethernet address of a bridge and its unique port addresses. This
field contains the ID for the perceived root bridge.
- Root Path Cost: A 4-byte field that contains a “cost” value
composed of individual bridge port costs along a data path. Bridges
use this information to determine the optimum frame transmission
path.
- Bridge Identifier: An 8-byte identification number that is derived
from the Ethernet address of a bridge and its unique port addresses.
- Port Identifier: A 2-byte field that contains port priority information
based on unique bridge port addresses. The port with the lowest
address has the highest priority.