mstp实例分析

拓扑：

 

 

SW1配置：

spanning-tree mode mst
spanning-tree extend system-id
!
spanning-tree mst configuration
name mst
revision 1
instance 1 vlan 10-19
instance 2 vlan 20-29
!
spanning-tree mst 1-2 priority 24576
!
interface Ethernet0/0
switchport
switchport trunk encapsulation dot1q
switchport mode trunk
spanning-tree link-type point-to-point
spanning-tree mst 0-2 cost 10
!
interface Ethernet0/1
switchport
switchport trunk encapsulation dot1q
switchport mode trunk
spanning-tree link-type point-to-point
spanning-tree mst 0-2 cost 10

SW2配置：

spanning-tree mode mst
spanning-tree extend system-id
!
spanning-tree mst configuration
name mst
revision 1
instance 1 vlan 10-19
instance 2 vlan 20-29
!
spanning-tree mst 1-2 priority 28672
!
interface Ethernet0/0
switchport
switchport trunk encapsulation dot1q
switchport mode trunk
spanning-tree link-type point-to-point
spanning-tree mst 0-2 cost 10
!
interface Ethernet0/1
switchport
switchport trunk encapsulation dot1q
switchport mode trunk
duplex auto
spanning-tree link-type point-to-point
spanning-tree mst 0-2 cost 10

SW3配置：

spanning-tree mode mst
spanning-tree extend system-id
!
spanning-tree mst configuration
name mst
revision 2
instance 1 vlan 10-19
instance 2 vlan 20-29
!
spanning-tree mst 1-2 priority 24576
!
interface Ethernet0/0
switchport
switchport trunk encapsulation dot1q
switchport mode trunk
duplex auto
spanning-tree link-type point-to-point
spanning-tree mst 0-2 cost 10
!
interface Ethernet0/1
switchport
switchport trunk encapsulation dot1q
switchport mode trunk
spanning-tree link-type point-to-point
spanning-tree mst 0-2 cost 10
!
interface Ethernet0/2
switchport
switchport trunk encapsulation dot1q
switchport mode trunk
spanning-tree link-type point-to-point
spanning-tree mst 0-2 cost 10

SW4配置：

spanning-tree mode mst
spanning-tree extend system-id
!
spanning-tree mst configuration
name mst
revision 2
instance 1 vlan 10-19
instance 2 vlan 20-29
!
interface Ethernet0/0
switchport
switchport trunk encapsulation dot1q
switchport mode trunk
spanning-tree link-type point-to-point
spanning-tree mst 0-2 cost 10
!
interface Ethernet0/1
switchport
switchport trunk encapsulation dot1q
switchport mode trunk
spanning-tree link-type point-to-point
spanning-tree mst 0-2 cost 10

SW5配置：

spanning-tree mode mst
spanning-tree extend system-id
!
spanning-tree mst configuration
name mst
revision 2
instance 1 vlan 10-19
instance 2 vlan 20-29
!
interface Ethernet0/0
switchport
switchport trunk encapsulation dot1q
switchport mode trunk
spanning-tree link-type point-to-point
spanning-tree mst 0-2 cost 10
!
interface Ethernet0/1
switchport
switchport trunk encapsulation dot1q
switchport mode trunk
spanning-tree link-type point-to-point
spanning-tree mst 0-2 cost 10
!
interface Ethernet0/2
switchport
switchport trunk encapsulation dot1q
switchport mode trunk
spanning-tree link-type point-to-point
spanning-tree mst 0-2 cost 10

SW6配置：

spanning-tree mode mst
spanning-tree extend system-id
!
spanning-tree mst configuration
name mst
revision 3
instance 1 vlan 10-19
instance 2 vlan 20-29
!
interface Ethernet0/0
switchport
switchport trunk encapsulation dot1q
switchport mode trunk
spanning-tree link-type point-to-point
spanning-tree mst 0-2 cost 10
!
interface Ethernet0/1
switchport
switchport trunk encapsulation dot1q
switchport mode trunk
spanning-tree link-type point-to-point
spanning-tree mst 0-2 cost 10

interface Ethernet0/2
switchport
switchport trunk encapsulation dot1q
switchport mode trunk
spanning-tree link-type point-to-point
spanning-tree mst 0-2 cost 10

SW7配置：

spanning-tree mode mst
spanning-tree extend system-id
!
spanning-tree mst configuration
name mst
revision 3
instance 1 vlan 10-19
instance 2 vlan 20-29
!
spanning-tree mst 1-2 priority 24576
!
interface Ethernet0/0
switchport
switchport trunk encapsulation dot1q
switchport mode trunk
spanning-tree link-type point-to-point
spanning-tree mst 0-2 cost 10
!
interface Ethernet0/1
switchport
switchport trunk encapsulation dot1q
switchport mode trunk
spanning-tree link-type point-to-point
spanning-tree mst 0-2 cost 10
!
interface Ethernet0/2
switchport
switchport trunk encapsulation dot1q
switchport mode trunk
spanning-tree link-type point-to-point
spanning-tree mst 0-2 cost 10

SW8配置：

spanning-tree mode mst
spanning-tree extend system-id
!
spanning-tree mst configuration
name mst
revision 3
instance 1 vlan 10-19
instance 2 vlan 20-29
!
interface Ethernet0/0
switchport
switchport trunk encapsulation dot1q
switchport mode trunk
spanning-tree link-type point-to-point
spanning-tree mst 0-2 cost 10
!
interface Ethernet0/1
switchport
switchport trunk encapsulation dot1q
switchport mode trunk
spanning-tree link-type point-to-point
spanning-tree mst 0-2 cost 10

SW9配置：

spanning-tree mode rapid-pvst
spanning-tree extend system-id
!
interface Ethernet0/0
switchport
switchport trunk encapsulation dot1q
switchport mode trunk
spanning-tree link-type point-to-point

分析：

区域A

 因为SW1的优先级（24577）小于SW2的优先级（28637）因此对于区域A来讲SW1是跟桥。

因为SW3的优先级大于SW4 SW5的优先级，因此在区域B中被选位跟桥。

因为SW7的优先级大于SW6 SW8的优先级，因此在区域C中被选位跟桥。

从上面分析能够看出在区域内的行为和RSTP相似。从这个交换机网络来讲无论是MST区域仍是非MST区域（SW9）选举跟桥（总根）的行为是同样的既最小的桥ID。

 

 

从上图看出若是把区域看作是一个虚拟的交换机，那个在整个区域的行为和RSTP相似。