20-2
STP identifies the network topology by transmitting BPDUs between STP compliant network devices,
typically switches and routers. BPDUs contain sufficient information for the network devices to complete
the spanning tree calculation.
In STP, BPDUs come in two types:
z
Configuration BPDUs, used to calculate spanning trees and maintain the spanning tree topology.
z
Topology change notification (TCN) BPDUs, used to notify concerned devices of network topology
changes, if any.
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 an entire STP-based network at a given time. But the root
bridge can change because of with changes of the network topology. Therefore, the root bridge is not
fixed.
Upon initialization of a network, each device generates and sends out BPDUs periodically with itself as
the root bridge; after network convergence, only the root bridge generates and sends out configuration
BPDUs at a certain interval, and the other devices just forward the BPDUs.
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 20-1
Designated bridge and designated port
Classification
Designated bridge
Designated port
For a device
A designated bridge is a device that is
directly connected to a switch and is
responsible for forwarding BPDUs to this
switch.
The port through which the
designated bridge forwards
BPDUs to this device
For a LAN
A designated bridge is a device
responsible for forwarding BPDUs to this
LAN segment.
The port through which the
designated bridge forwards
BPDUs to this LAN segment
Figure 20-1
shows designated bridges and designated ports. In the figure, AP1 and AP2, BP1 and BP2,
and CP1 and CP2 are ports on Device A, Device B, and Device C respectively.
z
If Device A forwards BPDUs to Device B through AP1, the designated bridge for Device B is Device
A, and the designated port is the port AP1 on Device A.
z
Two devices are connected to the LAN: Device B and Device C. If Device B forwards BPDUs to the
LAN, the designated bridge for the LAN is Device B, and the designated port is the port BP2 on
Device B.
