Signamax Managed Hardened PoE Industrial DIN-rail Mount Switch - Manual (Page 58)

11 Spanning Tree
53
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
The following figure 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.
 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.
 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.
(4) Bridge ID
A bridge ID consists of eight bytes, where the first two bytes represent the bridge priority of
the device, and the latter six bytes represent the MAC address of the device.
(5) Path cost
STP uses path costs to indicate the quality of links. A small path cost indicates a higher link
quality. The path cost of a port is related to the rate of the link connecting the port. The higher
the link rate, the smaller the path cost.
By comparing the path costs of different links, STP selects the most robust links and blocks