H3C Technologies H3C S7500E Series Switches - Manual (Page 154)

13-2
In the narrow sense, STP refers to IEEE 802.1d STP; in the broad sense, STP refers to the IEEE
802.1d STP and various enhanced spanning tree protocols derived from that protocol.
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:
Configuration BPDUs, used for calculating a spanning tree and maintaining the spanning tree
topology.
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 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 configuration 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 13-1 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 13-1 , AP1 and AP2, BP1 and BP2, and CP1 and CP2 are ports on Device A,
Device B, and Device C respectively.