•
Hello time: Configuration BPDU transmission interval.
•
Forward delay: Delay that STP bridges use to transition port state.
Basic concepts in STP
Root bridge
A tree network must have a root bridge. The entire network contains only one root bridge. The root
bridge is not permanent, but can change along with changes of the network topology.
Upon initialization of a network, each device generates and periodically sends configuration BPDUs,
with itself as the root bridge. After network convergence, only the root bridge generates and periodically
sends configuration BPDUs, and the other devices forward the BPDUs.
Root port
On a non-root bridge, the port nearest to the root bridge is the root port. The root port is responsible for
communication with the root bridge. Each non-root bridge has only one root port. The root bridge has no
root port.
Designated bridge and designated port
Table 7
Description of designated bridges and designated ports
Classification
Designated bridge
Designated port
For a device
A device directly connected to 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
, both Device B and Device C directly connect to the LAN. If Device A forwards
BPDUs to Device B through port A1, the designated bridge for Device B is Device A, and the designated
port of Device B is port A1 on Device A. 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 port B2 on Device B.
Figure 11
A schematic diagram of designated bridges and designated ports
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