Basic Concepts - Huawei AR530 series Configuration Manual

Huawei AR530&AR550 Series Industrial Switch Routers
Configuration Guide - Ethernet Switching
l

6.2.2 Basic Concepts

One Root Bridge
A tree topology must have a root. As defined in STP, the device that functions as the root of a
tree network is called the root bridge.
There is only one root bridge on the entire STP network. The root bridge is the logical center of
but is not necessarily at the physical center of the network. The root bridge changes dynamically
with the network topology.
After network convergence completes, the root bridge generates and sends configuration BPDUs
at specific intervals. Other devices process and forward the configuration BPDU to communicate
the topology changes, ensuring a stable network topology.
Two Metrics
A spanning tree is calculated based on two metrics: ID and path cost.
l
l
Issue 01 (2014-11-30)
2 and forward the request through port 1. As this process repeats, resources on the entire
network are exhausted, and the network finally breaks down.
Assume that no broadcast storm has occurred on the network shown in
HostA sends a unicast packet to HostB. If HostB is temporarily removed from the network
at this time, the MAC address entry for HostB on S1 and S2 are deleted. The unicast packet
sent by HostA to HostB is received by port 1 on S1. S1 has no matching MAC address
entry, so the unicast packet is forwarded to port 2. Then port 2 on S2 receives the unicast
packet from port 2 on S1 and sends it out through port 1. In addition, port 1 on S2 also
receives the unicast packet sent by HostA to HostB, and sends it out through port 2. As
such transmissions repeat, port 1 and port 2 on S1 and S2 continuously receive unicast
packets from HostA. S1 and S2 modify the MAC address entries continuously, causing the
MAC address table to flap. As a result, MAC address entries are damaged.
ID
IDs are classified into bridge ID (BID) and port ID (PID).
– BID
According to IEEE 802.1D, a BID is composed of a bridge priority (leftmost 16 bits)
and a bridge MAC address (rightmost 48 bits).
On an STP network, the device with the smallest BID is elected as the root bridge.
– PID
A PID is composed of a port priority (leftmost 4 bits) and a port number (rightmost 12
bits).
The PID is used to select the designated port.
NOTE
The port priority affects the role of a port in a specified spanning tree instance. For details, see
6.2.4 STP Topology
Path cost
The path cost is a port variable used for link selection. STP calculates path costs to select
robust links and blocks redundant links, and finally trims the network into a loop-free tree
topology.
Huawei Proprietary and Confidential
Copyright © Huawei Technologies Co., Ltd.
Calculation.
6 STP/RSTP Configuration
Figure 6-1.
164