Specifications

Ethernet Bridge Operation 9-7
Spanning Tree Operation
The primary function of the Spanning Tree Algorithm is to ensure that
there is only one data path between any two end stations within the
bridged Local Area Network. All computations are geared towards the
fact that a bridge wants to be considered as the Designated Bridge for any
LAN that it is connected to. Upon power up, BPDUs are sent out as
multicast frames. A bridge directly connected to the same LAN that the
BPDUs are sent out on will accept these BPDUs and make decisions on
their contents.
For ease of explanation we will represent the individual bridge’s BPDUs
in a shortened version consisting of four parameters as shown below:
• Bridge ID: A 64-bit value that is comprised of two individual
components:
- Bridge Priority: A 16-bit configured value
- MAC Address: A 48-bit value which is the hardware Ethernet
address of the bridge, e.g., 00-00-1d-e2-45-a2
• Root Bridge ID: Same as the Bridge ID but is the ID of the designated
root bridge.
• Path Cost: A value representing the contributing cost of passing
through this bridge. The formula used to determine the default is
1000/network Mbps per sec. The default for Ethernet is 1000/10=100.
• Port ID: A 16-bit value made up of two components:
- Port Priority: 8 bits in length and is the most significant byte of the
Port ID.
- Port ID: 8 bits in length numbered sequentially on a bridge from 1
to infinity (in theory).
The default Port ID for port 1 resembles the following: 8000-0001.
BPDU
Root Bridge ID
Path Cost
Bridge ID
Port ID