Configuring Spanning Tree Parameters Spanning Tree Overview
OmniSwitch AOS Release 8 Network Configuration Guide December 2017 page 6-7
Bridge Protocol Data Units (BPDU)
Switches send layer 2 frames, referred to as Configuration Bridge Protocol Data Units (BPDU), to relay
information to other switches. The information in these BPDU is used to calculate and reconfigure the
Spanning Tree topology. A Configuration BPDU contains the following information that pertains to the
bridge transmitting the BPDU:
The sending and receiving of Configuration BPDU between switches participating in the bridged network
constitute the root bridge election; the best path to the root is determined and then advertised to the rest of
the network. BPDU provide enough information for the STP software running on each switch to determine
the following:
• Which bridge serves as the root bridge.
• The shortest path between each bridge and the root bridge.
• Which bridge serves as the designated bridge for the LAN.
• Which port on each bridge serves as the root port.
• The port state (forwarding or discarding) for each bridge port based on the role the port plays in the
active Spanning Tree topology.
The following events trigger the transmitting and/or processing of BPDU in order to discover and
maintain the Spanning Tree topology:
• When a bridge first comes up, it assumes it is the root and starts transmitting Configuration BPDU on
all its active ports advertising its own bridge ID as the root bridge ID.
• When a bridge receives BPDU on its root port that contains more attractive information (higher
priority parameters and/or lower path costs), it forwards this information on to other LANs to which it
is connected for consideration.
Root ID The Bridge ID for the bridge that this bridge believes is the root.
Root Path Cost The sum of the Path Costs that lead from the root bridge to this bridge port.
The Path Cost is a configurable parameter value. The IEEE 802.1D standard specifies a
default value that is based on port speed. See “Configuring Port Path Cost” on
page 6-36 for more information.
Bridge ID An eight-byte hex value that identifies this bridge within the Spanning Tree. The first
two bytes contain a configurable priority value and the remaining six bytes contain a
bridge MAC address. See “Configuring the Bridge Priority” on page 6-28 for more
information.
Each switch chassis is assigned a dedicated base MAC address. This is the MAC
address that is combined with the priority value to provide a unique Bridge ID for the
switch. For more information about the base MAC address, see the appropriate
Hardware Users Guide for the switch.
Port ID A 16-bit hex value that identifies the bridge port that transmitted this BPDU. The first 4
bits contain a configurable priority value and the remaining 12 bits contain the physical
switch port number. See “Configuring Port Priority” on page 6-35 for more
information.
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