Bridge Protocol Data Units; Determining The Path For Forwarding Bpdus; Bridge Priority - Lenovo ThinkSystem NE2552E Application Manual

Bridge Protocol Data Units

Determining the Path for Forwarding BPDUs

Bridge Priority

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NE2552E Application Guide for ENOS 8.4
To create a Spanning Tree, the switch generates a configuration Bridge Protocol
Data Unit (BPDU), which it then forwards out of its ports. All switches in the Layer
2 network participating in the Spanning Tree gather information about other
switches in the network through an exchange of BPDUs.
A bridge sends BPDU packets at a configurable regular interval (2 seconds by
default). The BPDU is used to establish a path, much like a hello packet in IP
routing. BPDUs contain information about the transmitting bridge and its ports,
including bridge MAC addresses, bridge priority, port priority, and path cost. If the
ports are tagged, each port sends out a special BPDU containing the tagged
information.
The generic action of a switch on receiving a BPDU is to compare the received
BPDU to its own BPDU that it will transmit. If the priority of the received BPDU is
better than its own priority, it will replace its BPDU with the received BPDU. Then,
the switch adds its own bridge ID number and increments the path cost of the
BPDU. The switch uses this information to block any necessary ports.
Note: If STP is globally disabled, BPDUs from external devices will transit the
switch transparently. If STP is globally enabled, for ports where STP is turned off,
inbound BPDUs will instead be discarded.
When determining which port to use for forwarding and which port to block, the
NE2552E uses information in the BPDU, including each bridge ID. A technique
based on the "lowest root cost" is then computed to determine the most efficient
path for forwarding.
The bridge priority parameter controls which bridge on the network is the STG
root bridge. To make one switch become the root bridge, configure the bridge
priority lower than all other switches and bridges on your network. The lower the
value, the higher the bridge priority. Use the following command to configure the
bridge priority:
NE2552E(config)# spanning-tree stp <STP instance or range> bridge priority
<0‐65535>