Using STP spanning tree protocol to solve the problem of two-layer loop in network

The interconnection between switches will produce a closed physical loop , This means that there may be more than one path between the two networks . These loops may be due to the need for redundant paths to ensure reliability , It may also be caused by mistakes . If there is a loop in the switch network , Sent to the host Broadcast 、Unknown Unicast Frame Will not be able to reach the destination , The network will also stop serving . That's what happened , It is mainly determined by two characteristics of transparent switches ：

When the destination address is not in its address table , Continue flooding ;
When forwarding frames , The copy cannot be distinguished from the original frame . No fields （ For example, jump counter ） To distinguish consecutive copies

that , How to solve this problem ？

There are two possible solutions , First, it is strictly forbidden to create networks with loops , The second is to enable some mechanism , Allow the switch to detect whether there is a loop in the topology , Thus, the necessary interfaces are disabled to avoid the generation of loops . This is the spanning tree protocol （STP,Spanning Tree Protocol） Where it works . There is only one possible path between any pair of nodes in the spanning tree （ A tree without cycles ）. The goal of the spanning tree protocol is to logically disable the interface to always implement the spanning tree .
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therefore , In order to understand the topology of the network and disable the interface , A spanning tree message will be sent , These messages are called BPDU（ Bridge protocol data unit ）.BPDU Use the reserved multicast address 01-80-C2-00-00-00-00, To ensure that non spanning tree switches transparently propagate them .
Operation flow of spanning tree protocol ：

• Switches regularly exchange with network topology information BPDU.
• Each switch has an identifier (sw-ID), It is based on its MAC Address creation .
• Besides , Each port has an identifier (port-ID) And the costs associated with it .
• Switch selection has the lowest sw-ID As the root of the tree . They all choose the same .
• Each switch sends BPDUs, It shows that sw-ID、 Root switch sw-ID And the cost of reaching the root switch .BPDU Spread throughout the network , When forwarding information, each switch will add the cost of the interface it passes .
• received BPDU after , Each switch will calculate which port it can pass through to reach the root switch at the lowest cost , Then determine the root port , At the same cost , It will choose the lowest port ID.
• Every LAN There is a designated port , This port is the LAN Access the port of the root switch at the lowest cost .
• Neither the root nor the specified port is the blocking port . These ports are unnecessary for communication , If operation is allowed , They cause a cycle .

Fast spanning tree protocol ：RSTP
Traditional spanning tree (IEEE 802.1D) There is a convergence problem , Because topology changes may require 30 Seconds to minutes . In some cases, this cannot meet the requirements of users .1998 year , Fast spanning tree protocol （RSTP,IEEE 802.1w） The standardization work is completed , This is a variant of the original protocol , The convergence time can be reduced to about 6 second . Now , The traditional spanning tree has been declared obsolete .
stay RSTP in , The switch retains information about the second lowest cost path to the root , Therefore, it is better to switch to the new topology in case of failure than STP Faster protocol .

Multiple RSTP：MSTP

RSTP The protocol has become an ideal choice to prevent loops in the layer-2 network in the topology including redundancy . If there are switches in the network , Then it must be able to run RSTP Algorithm .
Another interesting form of redundancy or security for loops is the use of multiple spanning tree protocols (MSTP).MSTP yes RSTP An extension of , Can be VLAN Group into spanning tree instances . It provides multiple forwarding paths for data traffic , And achieve load balancing .
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MSTP For each VLAN The group is configured with a separate spanning tree , Allow each VLAN There is a configured root bridge and forwarding topology . It blocks all possible alternate paths in each spanning tree , Keep only one .
Hongke provides RSTP and MSTP Technology of integrating functions into the network , Hongke network management redundant switch （MES）. This is a with many configuration options FPGA Of IP Nuclear solutions , Able to support RSTP and MSTP, Switches that can be used in many types of applications . Want to know more about it STP Technology and application of the Protocol , Please feel free to pass [email protected] Contact us .