F3726, F3211, F3174, R5135, R3816-HP Firewalls and UTM Devices Network Management Configuration Guide-6PW100

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411BBasic concepts in STP
1110BRoot bridge
A tree network must have a root bridge.
There is only one root bridge in the entire network. The entire network contains only one root bridge. All
the other bridges in the network are called "leaf nodes." The root bridge is not permanent, but can
change when the network topology changes.
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. The other devices only forward BPDUs.
1111BRoot port
On a non-root bridge, the port nearest to the root bridge is the root port. The root port communicates with
the root bridge. Each non-root bridge has only one root port. The root bridge has no root port.
1112BDesignated bridge and designated port
Table 7 Description of designated bridges and designated ports
Classification Desi
g
nated brid
g
e Desi
g
nated
p
ort
For a device
Device directly connected with the local
device and responsible for forwarding
BPDUs to the local device.
Port through which the designated bridge
forwards BPDUs to this device.
For a LAN
Device responsible for forwarding
BPDUs to this LAN segment.
Port through which the designated bridge
forwards BPDUs to this LAN segment.
As shown in 2195HFigure 39, Device B and Device C are directly connected to a 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 39 Schematic diagram of designated bridges and designated ports
1113BPath cost
Path cost is a reference value used for link selection in STP. STP calculates path costs to select the most
robust links and block redundant links that are less robust, to prune the network into a loop-free tree.