Spanning Tree protocols in a hierarchical network

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Questions derived from the 642-812 – Building Converged Cisco Multilayer Switched Networks (BCMSN) Cisco Self Test Software Practice Test.


Objective: Conduct the operation of Spanning Tree protocols in a hierarchical network
SubObjective: Configure RSTP (PVRST) and MISTP


Item Number: 642-812.2.2.3
Single Answer, Multiple Choice


Which command maps VLANs 1-10 and 15 to MST instance 2?



  1. Switch(config)# instance 2 vlan 1-10,15
  2. Switch(config-mst)# instance 2 vlan 1-10,15
  3. Switch(config)# spanning-tree mode mst 1-10,15
  4. Switch(config)# spanning-tree mode mst 2 vlan 1-10,15


B. Switch(config-mst)# instance 2 vlan 1-10,15


Use the instance command in MST configuration mode to map a VLAN (or range of VLANs) to an instance. The proper syntax is instanceinstance_numbervlanvlan_range; for example:


Switch(config-mst)# instance 2 vlan 1-10,15


Multiple Spanning Tree (MST) 802.1s enhances the 802.1Q common spanning tree (CST) by allowing one spanning tree instance per administrative grouping of VLANs instead of one instance for all VLANs. Once the desired topology is determined, VLANs of similar topology are mapped into a group and assigned to an MST spanning tree instance. This reduces the overhead of maintaining individual spanning tree instances for each VLAN of redundant topologies while allowing better load distribution by using redundant links. MST is advertised to have the fast-convergence benefits of RSTP and the load-balancing enhancements of Cisco’s PVST+.


802.1s describes MST regions made up of groups of switches. A region is a group of MST switches under one administrative control. VLANs within the region are mapped to one of sixteen possible spanning tree instances. Instance 0 is the default and always present.


All switches running MST within the region have the following attributes in common:



  • Region Name (a 32-character string that identifies the MST region)
  • Revision Number (a 16-bit number that identifies the current configuration revision, incremented manually)
  • VLAN table (an array of 4,096 bytes where each integer equates to a VLAN and the instance it is associated with)




Each switch within the region supports one MST instance of spanning tree referred to as Internal Spanning Tree (IST). This IST connects all the bridges within the region and appears as a single bridge to those outside of the MST region. Fifteen more spanning tree instances can be defined within the region and VLANs can be assigned to them. For example:


Switch(config)#spanning-tree mode mst
Switch(config)#spanning-tree mst configuration
Switch(config-mst)#name REGION
Switch(config-mst)#revision 2
Switch(config-mst)#instance 2 vlan 10
Switch(config-mst)#show pending


Pending MST configuration
Revision 2
Instance Vlans mapped
—————– ———————————————————-
0 1-9, 11-4094
2 10




The Switch(config-mst)# end command applies the configuration and exits.



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