On an STP network, a port's accumulative path cost to the root bridge is the sum of the path
costs of all ports between the port and the root bridge. The accumulative path cost is the
root path cost.
Three Elements
Three elements are involved in pruning a ring network into a tree network: root bridge, root port,
and designated port. Figure 6-2 shows the three elements.
Figure 6-2 STP network architecture
S1
S2
S3
S4
root
bridge
A
PC=100;RPC=0
B
PC=100;RPC=100
A
PC=99;RPC=199
B
PC=100;RPC=0
A
PC=100;RPC=100
B
PC=200;RPC=300
A
PC=200;RPC=300
B
PC=99;RPC=199
PC: path cost
RPC: root path cost
root port
designated port
blocked port
l Root bridge
The root bridge is the bridge with the smallest BID. The smallest BID is discovered by
exchanging configuration BPDUs.
l Root port
The root port on an STP device is the port with the smallest path cost to the root bridge and
is responsible for forwarding data to the root bridge. The root port is determined based on
root path costs of all ports. Among all the STP ports on a device, the port with the smallest
root path cost is the root port. An STP device has only one root port, and there is no root
port on the root bridge.
l Designated port
Table 6-2 explains what designated bridge and designated port are.
Huawei AR530&AR550 Series Industrial Switch Routers
Configuration Guide - Ethernet Switching
6 STP/RSTP Configuration
Issue 01 (2014-11-30) Huawei Proprietary and Confidential
Copyright © Huawei Technologies Co., Ltd.
165
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