250
SwitchD:
SwitchD(Config)#vlan 20
SwitchD(Config-Vlan20)#exit
SwitchD(Config)#vlan 30
SwitchD(Config-Vlan30)#exit
SwitchD(Config)#vlan 40
SwitchD(Config-Vlan40)#exit
SwitchD(Config)#vlan 50
SwitchD(Config-Vlan50)#exit
SwitchD(Config)#spanning-tree mst configuration
SwitchD(Config-Mstp-Region)#description mstp
SwitchD(Config-Mstp-Region)#instance 3 vlan 20;30
SwitchD(Config-Mstp-Region)#instance 4 vlan 40;50
SwitchD(Config-Mstp-Region)#exit
SwitchD(Config)#interface e1/1-7
SwitchD(Config-Port-Range)#switchport mode trunk
SwitchD(Config-Port-Range)#exit
SwitchD(Config)#spanning-tree
SwitchD(Config)#spanning-tree mst 4 priority 0
After the above configuration, SwitchA is the root bridge of the instance 0 of the
entire network. In the MSTP region which SwitchB, SwitchC and SwitchD belong to,
SwitchB is the region root of the instance 0, SwitchC is the region root of the instance 3
and SwitchD is the region root of the instance 4. The traffic of VLAN 20 and VLAN 30 is
sent through the topology of the instance 3. The traffic of VLAN 40 and VLAN 50 is sent
through the topology of the instance 4. And the traffic of other VLANs is sent through the
topology of the instance 0. The port 1 in SwitchB is the master port of the instance 3 and
the instance 4.
The MSTP calculation generates 3 topologies: the instance 0, the instance 3 and the
instance 4 (marked with blue lines). The ports with the mark “x” are in the status of
discarding. The other ports are the status of forwarding. Because the instance 3 and the
instance 4 are only valid in the MSTP region, the following figure only shows the topology
of the MSTP region.
