H3C S3100-52P Operation Manual page 246

Operation Manual – MSTP
H3C S3100-52P Ethernet Switch
II. Protocol packets of STP
STP uses bridge protocol data units (BPDUs), also known as configuration messages,
as its protocol packets.
STP identifies the network topology by transmitting BPDUs between STP compliant
network devices. BPDUs contain sufficient information for the network devices to
complete the spanning tree calculation.
In STP, BPDUs come in two types:
Configuration BPDUs, used to calculate spanning trees and maintain the spanning
tree topology.
Topology change notification (TCN) BPDUs, used to notify concerned devices of
network topology changes, if any.
III. Basic concepts in STP
1) Root bridge
A tree network must have a root; hence the concept of root bridge has been
introduced in STP.
There is one and only one root bridge in the entire network, and the root bridge can
change alone with changes of the network topology. Therefore, the root bridge is not
fixed.
Upon network convergence, the root bridge generates and sends out configuration
BPDUs periodically. Other devices just forward the configuration BPDUs received. This
mechanism ensures the topological stability.
2) Root port
On a non-root bridge device, the root port is the port with the lowest path cost to the root
bridge. The root port is used for communicating with the root bridge. A non-root-bridge
device has one and only one root port. The root bridge has no root port.
3) Designated bridge and designated port
Refer to the following table for the description of designated bridge and designated
port.
Table 1-1 Designated bridge and designated port
Classification
For a device
Designated bridge
A designated bridge is a device
that is directly connected to a
switch and is responsible for
forwarding BPDUs to this
switch.
1-2
Chapter 1 MSTP Configuration
Designated port
The port through which
the designated bridge
forwards BPDUs to this
device