1-2
Protocol Packets of STP
STP uses bridge protocol data units (BPDUs), also known as configuration messages, as its protocol
packets.
STP-enabled network devices exchange BPDUs to establish a spanning tree. BPDUs contain sufficient
information for the network devices to complete spanning tree calculation.
In STP, BPDUs come in two types:
z
Configuration BPDUs, used for calculating a spanning tree and maintaining the spanning tree
topology.
z
Topology change notification (TCN) BPDUs, used for notifying the concerned devices of network
topology changes, if any.
Basic Concepts in STP
Root bridge
A tree network must have a root; hence the concept of root bridge was introduced in STP.
There is one and only one root bridge in the entire network, and the root bridge can change along 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.
Root port
On a non-root bridge, the port nearest to the root bridge is called the root port. The root port is
responsible for communication with the root bridge. Each non-root bridge has one and only one root
port. The root bridge has no root port.
Designated bridge and designated port
The following table describes designated bridges and designated ports.
Table 1-1
Description of designated bridges and designated ports:
Classification
Designated bridge
Designated port
For a device
A device directly connected with the local
device and responsible for forwarding
BPDUs to the local device
The port through which the
designated bridge forwards BPDUs
to this device
For a LAN
The device responsible for forwarding
BPDUs to this LAN segment
The port through which the
designated bridge forwards BPDUs
to this LAN segment
As shown in
Figure 1-1
, 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 of Device B is 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 for the LAN is the port
BP2 on Device B.
