多生成树可以实现链路的防环、冗余备份、负载均衡功能

拓朴如下：

主要配置如下：

[R1]
interface GigabitEthernet0/0/1
 ip address 10.1.1.254 255.255.255.0 
#
interface GigabitEthernet0/0/2
 ip address 10.2.2.254 255.255.255.0 
#

[S1]
interface GigabitEthernet0/0/23
 port link-type trunk
 port trunk allow-pass vlan 10 20
#
interface GigabitEthernet0/0/24
 port link-type trunk
 port trunk allow-pass vlan 10 20
#

[S2]
stp instance 1 root primary
stp instance 2 root secondary
#
stp region-configuration
 region-name 12
 revision-level 5
 instance 1 vlan 10
 instance 2 vlan 20
 active region-configuration
#

[S3]
stp instance 1 root secondary
stp instance 2 root primary
#
interface GigabitEthernet0/0/22
 port link-type trunk
 port trunk allow-pass vlan 10 20
#
interface GigabitEthernet0/0/23
#
interface GigabitEthernet0/0/24
 port link-type trunk
 port trunk allow-pass vlan 10 20
#

查证：

        可以看到：S2是实例1，也就是vlan 10的根桥，S2是实例2，vlan 20的根桥，S1上实例1和实例2，两个端口的角色分别为实例2和实例1的Alter角色，起到了负载均衡的作用，如果一根链路down掉后，另外一根链路起到了备份链路的作用。