Huawei AR530 Series - Page 188

The root ID is the BID of the local bridge, the root path cost is 0 (accumulative path cost
from the port to the local bridge), the sender BID is the BID of the local bridge, and the
PID is the ID of the port that sends the BPDU.
2. Root bridge election
During network initialization, every device considers itself the root bridge and sets the root
ID to its own BID. Then devices exchange configuration BPDUs and compare their root
IDs to find the device with the smallest BID, which finally becomes the root bridge.
3. Root port and designated port selection
Table 6-8 describes the process of selecting the root port and designated port.
Table 6-8 Selecting the root port and designated port
Ste
p
Process
1 A non-bridge device selects the port that receives the optimal configuration BPDU
as the root port. Table 6-9 describes the process of selecting the optimal
configuration BPDU.
2 The device generates a configuration BPDU for each port and calculates the fields
in the configuration BPDU based on the configuration BPDU on the root port and
path cost of the root port:
l Replaces the root ID with the root ID in the configuration BPDU on the root
port.
l Replaces the root path cost with the sum of the root path cost in configuration
BPDU on the root port and the path cost of the root port.
l Replaces the sender BID with the local BID.
l Replaces the PID with the local port ID.
3 The device compares the calculated configuration BPDU with the configuration
BPDU received on the port:
l If the calculated configuration BPDU is superior, the port is selected as the
designated port and periodically sends the calculated configuration BPDU.
l If the port's own configuration BPDU is superior, the configuration BPDU on
the port is not updated and the port is blocked. After that, the port only receives
BPDUs, and does not forward data or send BPDUs.
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.
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