Stp/Rstp Overview - Huawei Quidway S7700 Configuration Manual - Ethernet

Quidway S7700 Smart Routing Switch
Configuration Guide - Ethernet

8.1 STP/RSTP Overview

STP is a management protocol on the data link layer. It is used to block redundant links on the
Layer 2 network and trim a network into a loop-free tree. RSTP is a refinement of STP and
introduces rapid convergence of the network topology.
8.1.1 STP/RSTP Overview
STP/RSTP is used to block redundant links on the Layer 2 network and trim a network into a
loop-free tree topology.
Introduction
On a complex network, loops are inevitable. With the requirement for network redundancy
backup, network designers tend to deploy multiple physical links between two devices, one of
which is the master and the others are the backup. Loops are likely or bound to occur in such a
situation.
Loops will cause broadcast storms, thereby exhausting network resources and paralyzing the
network. Loops also cause flapping of MAC address tables and thus damages MAC address
entries.
The devices running STP discover loops on the network by exchanging information with each
other and trim the ring topology into a loop-free tree topology by blocking a certain interface.
In this manner, replication and circular propagation of packets are prevented on the network. In
addition, it is prevented that the processing performance of devices is degraded when
continuously processing repeated packets.
STP, however, converges the network topology slowly. In 2001, the IEEE published document
802.1w to introduce an evolution of the Spanning Tree Protocol: Rapid Spanning Tree Protocol
(RSTP). RSTP is developed based on STP but outperforms STP.
Concepts
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Root bridge
A tree topology must have a root. Therefore, the root bridge is introduced by STP/RSTP.
There is only one root bridge on the entire STP/RSTP-capable network. The root bridge is
the logical center but is unnecessarily the physical center of the entire network. The root
bridge may be served by another switching device along with the network topology change.
ID
There are Bridge IDs (BIDs) and port IDs (PIDs).
– BID
IEEE 802.1D defines that a BID is composed of a 2-bit bridge priority and a bridge
MAC address. That is, BID (8 bits) = Bridge priority (2 bits) + Bridge MAC address (6
bits).
On the STP-capable network, the device with the smallest BID is selected as the root
bridge. The bridge priority that is allowed to be configured on a Huawei device ranges
from 0 to 61440. By default, the bridge priority is 32768.
– PID
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