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RSTP在STP基础上做了改进，实现了网络拓扑快速收敛，但RSTP和STP还存在同一个缺陷，即由于局域网内所有的VLAN共享一颗生成树，链路被阻塞后将不承载任何流量，造成带宽浪费，因此无法在VLAN间实现数据流量的负载均衡，还有可能造成部分VLAN报文无法转发。
通过MSTP把一个交换网络化分成多个域，每个域内形成多颗生成树，生成树之间彼此独立，每个域叫做MSTP域（Multiple Spanning Tree Region MST Region），每棵生成树叫做一个多生成树实例MSTI（Multiple Spanning Tree Instance）
实例内可以包含多个VLAN。通过多个VLAN映射到同一个实例内，可以节省通信开销和资源占用率，MSTP各个实例拓扑的生成树计算相互独立，通过这些实例可以实现负载均衡，把多个相同拓扑结构的VLAN映射到一个实例里，这些VLAN在端口上转发状态取决于端口在对应MSTP实例的状态
MSTP通过设置VLAN映射表（即VLAN和MSTI的对应关系表），把VLAN和MSTI联系起来，每个VLAN只能对应一个MSTI，即同一个VLAN的数据只能在一个MSTI中传输，而一个MSTI可能对应多个VLAN。
| 设备 | 接口 | ip | 子网 | 默认网关 |
| PC1 | Ethernet0/0/1 | 192.168.10.1 | 255.255.255.255.0 | |
| PC2 | Ethernet0/0/1 | 192.168.20.1 | 255.255.255.255.0 | |
| PC3 | Ethernet0/0/1 | 192.168.10.2 | 255.255.255.255.0 | |
| PC4 | Ethernet0/0/1 | 192.168.20.2 | 255.255.255.255.0 |
| 设备 | 全局MAC地址 |
| SW1 | 4c1f-cc92-6fe3 |
| SW2 |
4c1f-cc11-5ec3 |
| SW3 |
4c1f-cc46-1924 |
SW1配置
vlan batch 10 20 interface Ethernet0/0/1 port link-type trunk port trunk allow-pass vlan 10 20 # interface Ethernet0/0/3 port link-type access port default vlan 10
SW2配置
vlan batch 10 20 interface Ethernet0/0/1 port link-type trunk port trunk allow-pass vlan 10 20 # interface Ethernet0/0/2 port link-type trunk port trunk allow-pass vlan 10 20 # interface Ethernet0/0/3 port link-type access port default vlan 20
SW3配置
vlan batch 10 20 interface Ethernet0/0/1 port link-type trunk port trunk allow-pass vlan 10 20 # interface Ethernet0/0/2 port link-type trunk port trunk allow-pass vlan 10 20 # interface Ethernet0/0/3 port link-type access port default vlan 10 # interface Ethernet0/0/4 port link-type access port default vlan 20
[SW1]dis stp -------[CIST Global Info][Mode MSTP]------- CIST Bridge :32768.4c1f-cc92-6fe3 Config Times :Hello 2s MaxAge 20s FwDly 15s MaxHop 20 Active Times :Hello 2s MaxAge 20s FwDly 15s MaxHop 20 CIST Root/ERPC :32768.4c1f-cc11-5ec3 / 200000 CIST RegRoot/IRPC :32768.4c1f-cc92-6fe3 / 0 CIST RootPortId :128.1 BPDU-Protection :Disabled TC or TCN received :10 TC count per hello :0 STP Converge Mode :Normal Time since last TC :0 days 0h:31m:39s Number of TC :4 Last TC occurred :Ethernet0/0/3 ----[Port1(Ethernet0/0/1)][FORWARDING]---- Port Protocol :Enabled Port Role :Root Port Port Priority :128 Port Cost(Dot1T ) :Config=auto / Active=200000 Designated Bridge/Port :32768.4c1f-cc11-5ec3 / 128.2 Port Edged :Config=default / Active=disabled Point-to-point :Config=auto / Active=true Transit Limit :147 packets/hello-time Protection Type :None Port STP Mode :MSTP Port Protocol Type :Config=auto / Active=dot1s BPDU Encapsulation :Config=stp / Active=stp PortTimes :Hello 2s MaxAge 20s FwDly 15s RemHop 20 TC or TCN send :3 TC or TCN received :7 BPDU Sent :4 TCN: 0, Config: 0, RST: 0, MST: 4 BPDU Received :888 TCN: 0, Config: 0, RST: 0, MST: 888
[SW1]dis stp brief MSTID Port Role STP State Protection 0 Ethernet0/0/1 ROOT FORWARDING NONE 0 Ethernet0/0/2 ALTE DISCARDING NONE 0 Ethernet0/0/3 DESI FORWARDING NONE [SW1] [SW2] dis stp brief MSTID Port Role STP State Protection 0 Ethernet0/0/1 DESI FORWARDING NONE 0 Ethernet0/0/2 DESI FORWARDING NONE 0 Ethernet0/0/3 DESI FORWARDING NONE [SW3] dis stp brief MSTID Port Role STP State Protection 0 Ethernet0/0/1 DESI FORWARDING NONE 0 Ethernet0/0/2 ROOT FORWARDING NONE 0 Ethernet0/0/3 DESI FORWARDING NONE 0 Ethernet0/0/4 DESI FORWARDING NONE
此时SW2交换机的所有端口都为指定端口，又都为转发状态，所以SW2为根交换机。SW1的e/0/02为阻塞口。
PC3pingPC1，PC4pingPC2，在SW2的E0/0/2上抓包。
可以观察到VLAN10、20的数据包都从SW2的E0/0/2转发，在SW1的e0/0/2上什么也抓不到，只抓到周期发送的BPDU
此时SW1和SW3之间的链路处理闲置状态，造成了资源浪费，
配置MSTP多实例。
[SW1]stp region-configuration 进入MSTP域视图 [SW1-mst-region]region-name huawei 配置MSTP域名为huawei [SW1-mst-region]revision-level 1 配置MSTP修订级别为1 [SW1-mst-region]instance 1 vlan 10 [SW1-mst-region]instance 2 vlan 20 指定VLAN10映射到MSTI1，指定VLAN20映射到MSTI2 [SW1-mst-region]active region-configuration 激话MSTP域配置
[SW1-mst-region]dis this # stp region-configuration region-name huawei revision-level 1 instance 1 vlan 10 instance 2 vlan 20 active region-configuration # return
SW2和SW3配置一样
SW1 SW2 SW3上使用
[SW1]dis stp region-configuration Oper configuration Format selector :0 Region name :huawei Revision level :1 Instance VLANs Mapped 0 1 to 9, 11 to 19, 21 to 4094 1 10 2 20
[SW2]dis stp region-configuration Oper configuration Format selector :0 Region name :huawei Revision level :1 Instance VLANs Mapped 0 1 to 9, 11 to 19, 21 to 4094 1 10 2 20
[SW3]dis stp region-configuration Oper configuration Format selector :0 Region name :huawei Revision level :1 Instance VLANs Mapped 0 1 to 9, 11 to 19, 21 to 4094 1 10 2 20
所有交换机上的MSTP域 名都为huawei，修定版本号都是1，且VLAN与实例的映射相同，VLAN10与20之外，其余VLAN属于实例0.
PC3持续PingPC1，PC4持续pingPC2。在SW2的E0/0/2上抓包。
可以观察到VLAN10、20的数据包都从SW2的E0/0/2转发，在SW1的e0/0/2上什么也抓不到，只抓到周期发送的BPDU
在SW1、SW2、SW3上使用
[SW1]dis stp instance 0 brief MSTID Port Role STP State Protection 0 Ethernet0/0/1 ROOT FORWARDING NONE 0 Ethernet0/0/2 ALTE DISCARDING NONE 0 Ethernet0/0/3 DESI FORWARDING NONE [SW2]dis stp instance 0 brief MSTID Port Role STP State Protection 0 Ethernet0/0/1 DESI FORWARDING NONE 0 Ethernet0/0/2 DESI FORWARDING NONE 0 Ethernet0/0/3 DESI FORWARDING NONE [SW3]dis stp instance 0 brief MSTID Port Role STP State Protection 0 Ethernet0/0/1 DESI FORWARDING NONE 0 Ethernet0/0/2 ROOT FORWARDING NONE 0 Ethernet0/0/3 DESI FORWARDING NONE 0 Ethernet0/0/4 DESI FORWARDING NONE
[SW1]dis stp instance 1 brief MSTID Port Role STP State Protection 1 Ethernet0/0/1 ROOT FORWARDING NONE 1 Ethernet0/0/3 DESI FORWARDING NONE [SW2]dis stp instance 1 brief MSTID Port Role STP State Protection 1 Ethernet0/0/1 DESI FORWARDING NONE 1 Ethernet0/0/2 DESI FORWARDING NONE [SW3]dis stp instance 1 brief MSTID Port Role STP State Protection 1 Ethernet0/0/1 DESI FORWARDING NONE 1 Ethernet0/0/2 ROOT FORWARDING NONE 1 Ethernet0/0/3 DESI FORWARDING NONE
[SW1]dis stp instance 2 brief MSTID Port Role STP State Protection 2 Ethernet0/0/1 ROOT FORWARDING NONE [SW2]dis stp instance 2 brief MSTID Port Role STP State Protection 2 Ethernet0/0/1 DESI FORWARDING NONE 2 Ethernet0/0/2 DESI FORWARDING NONE 2 Ethernet0/0/3 DESI FORWARDING NONE [SW3]dis stp instance 2 brief MSTID Port Role STP State Protection 2 Ethernet0/0/1 DESI FORWARDING NONE 2 Ethernet0/0/2 ROOT FORWARDING NONE 2 Ethernet0/0/4 DESI FORWARDING NONE
以 上三种实例选举结果一致，
手动指定SW1在实例2里为根交换机，
[SW1]stp instance 2 root primary
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