3Com Switch 8800 Advanced Software V5 Configuration Guide

94 CHAPTER 11: MSTP CONFIGURATION
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.
An STP network has only one root bridge. The root bridge is globally significant in
the entire network, and is the logical center of the network. However, it need not
be the physical center of the network. The root bridge may change when the
network topology changes. After the network converges, only the root bridge
sends out protocol packets known as configuration BPDUs at a specific interval,
and the other devices just relay these configuration BPDUs to keep the topology
stable.
2 Root port
In an STP network, a root port is a port on a non-root bridge device. Among the
ports on an STP-enabled device, the root port has the lowest path cost to the root
bridge.
The root port takes charge of 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 Table 8 for the description of designated bridge and designated port.
Tabl e 8 Description of designated bridge and designated port
Classification Designated bridge Designated port
For a device The device directly connected
with this device and
responsible for forwarding
configuration BPDUs
The port through which the
designated bridge forwards
configuration BPDUs to this
device
For a LAN The device responsible for
forwarding configuration
BPDUs to this LAN segment
The port through which the
designated bridge forwards
configuration BPDUs to this
LAN segment