Multi-Spanning Tree Protocol allows for resource conservation by now requring an instance for every single VLAN in the layer 2 network. This lab will discuss and demonstrate the configuration and verification of 802.1s MSTP.
If you have read through or completed Labs 4-13 and 4-15 then you should be quite familiar with spanning tree by now and understand that it prevents layer two network loops in a switching topology. This lab will take you’re understanding of spanning tree just a bit further. This lab will explain the function(s) of the IEEE Standard Multiple Spanning Tree protocol also referred to as MST.
MST is commonly used in extremely large networks where running PVST would tax switch resources. Multiple Spanning Tree Protocol gives you the ability to configure a hierarchical spanning tree topology where you have a common spanning tree instance known as the (CIST), technically instance 0 in MST Configuration mode.
MST is designed in a way that you’ll have regional STP instances. A region is defined by the name given in MST configuration mode. For example a region can be Boston or Miami.
Also per each region you have a revision number. This is a locally significant number to signify a revision for the MST configuration. Note that the name, instance map and revision number MUST MATCH in order to build a converged MST topology.
With MST you’re able utilize multiple core switches at a single location (region) by using different instances. For example; core switch 1 is the root bridge for all odd VLANs such as 1, 3, 5, 7 and so on. Core switch 2 is the root bridge for even VLANs such as 2,4,5,8,10 and so on.
Keep in mind when designing a core network for a particular region some VLAN’s may be more traffic intensive, so you may need to further load balance them out across multiple core switch instances in a given region.
MSTP and Rapid-PVST are inter-compatible however only instance 0 (the CIST) is shared from MST to Rapid-PVST.
MSTP configuration is quite different from PVST/Rapid-PVST in such that MSTP has its own configuration mode. In this mode you assign VLANs to a spanning tree instance then you can assign the instance as the root bridge by using the spanning-tree vlan # root primary command.
The port modes remain the same in MSTP as they were in RSTP which are shown below;
| Command | Description |
|---|---|
| root | The port that receives the best BPDU that is closest to the root bridge in terms of path cost is called the root port. The root bridge is the only bride in the network that does not have a root port. |
| designated | A port is designated if it can send the best BPDU on the segment to which it is directly connected. On a given LAN segment there can only be a single path towards the root bridge. This port forwards traffic to the LAN segment. Access ports are considered designated ports. |
| alternate | An alternate port is the next best path available back to the root bridge shall the root port fail. |
| backup | A backup port is a port that is connected to a segment where another bridge port already connects. |
In this lab you’ll use SW1 and SW2 to simulate a core and SW3 will be a distribution switch. SW1 will be the root bridge for VLANs 1,3,5,7,9 and SW2 will be the root bridge for VLANs 2,4,6,8,10. SW3 will be running Rapid-PVST.
In this lab you’ll familiarize yourself with the following NEW commands;
| Command | Description |
|---|---|
| spanning-tree mode mst | This command is executed in global configuration and configures the switch to use the IEEE 802.1s Multiple Spanning Tree Protocol (MSTP). |
| spanning-tree mst configuration | This command is executed in global configuration mode and places you into MST (Multiple Spanning Tree) configuration mode. |
| instance # vlan # | This command is executed from within MST configuration mode and configures an MST instance number and associated VLANs running on the MST instance. |
| name namegoeshere | This command is executed from within MST configuration mode and configures the region name for the MSTP switch. |
| spanning-tree instance# priority # | aThis command when executed from global configuration configures a specific instance of MST as the root bridge for the VLANs included in that MST instance. |
| show spanning-tree mst configuration | This command is executed in privileged mode to display the current MST configuration on a switch. |
| show spanning-tree mst # | This command is executed in privileged mode to display information such as the root bridge, root bridge mac address, root bridge priority, root port and other information on a per instance basis. |
| show spanning-tree mst detail | This command is executed in privileged mode to display detailed MST configuration on a switch such as root bridge information, MST bridge priority, interface participation, timers, port states, port cost and more. |
Due to the limited feature support of the NM-16ESW, this lab CANNOT be completed using the Free CCNA Workbook GNS3 topology. This lab can however be completed using the Stub Lab.
Step 1. – Configure SW1 to run MST using the revision number 1 and region name REGION1 then configure VLANs 1, 3, 5, 7 and 9 to run on SW1 MST instance 1. Configure SW1 as the root bridge of VLANs 1, 3, 5, 7 and 9 using the static priority of 8192.
To configure SW1 to run MSTP you’ll use the spanning-tree mode mst in global configuration mode. To configure MST instances on SW1 first you need to navigate to MST configuration mode by using the spanning-tree mst configuration command then the instance # vlan # command to create a new instance and map VLANs to that instance. The root bridge is configured on a per instance basis by using the command spanning-tree mst instance# priority # as shown below;
SW1 con0 is now available Press RETURN to get started. SW1>enable SW1#configure terminal Enter configuration commands, one per line. End with CNTL/Z. SW1(config)#spanning-tree mode mst SW1(config)#spanning-tree mst configuration SW1(config-mst)#revision 1 SW1(config-mst)#name REGION1 SW1(config-mst)#instance 1 vlan 1,3,5,7,9 SW1(config-mst)#exit SW1(config)#spanning-tree mst 1 priority 8192 SW1(config)#end %SYS-5-CONFIG_I: Configured from console by console SW1#
Step 2. – Configure SW1 to run VLANs 2, 4, 6, 8 and 10 on MST instance 2 then configure SW1 as the secondary root bridge for VLANs 2, 4, 6, 8 and 10 using the static priority of 16384.
To complete this step you’ll use the same commands as previously shown in the step 1 configuration example and as shown below;
SW1#configure terminal Enter configuration commands, one per line. End with CNTL/Z. SW1(config)#spanning-tree mst configuration SW1(config-mst)#instance 2 vlan 2,4,6,8,10 SW1(config-mst)#exit SW1(config)#spanning-tree mst 2 priority 16384 SW1(config)#end %SYS-5-CONFIG_I: Configured from console by console SW1#
Step 3 – Configure SW2 to run MST using the revision number 1 and region name REGION1 then configure VLANs 1, 3, 5, 7 and 9 to run on SW2 MST instance 1. Configure SW2 the secondary root bridge of VLANs 1, 3, 5, 7 and 9 using the static priority of 16384.
Now mirror the inverse configuration that you’ve done in steps 1 and 2 on SW2 in steps 3 and 4 as shown below;
SW2 con0 is now available Press RETURN to get started. SW2>enable SW2#configure terminal Enter configuration commands, one per line. End with CNTL/Z. SW2(config)#spanning-tree mode mst SW2(config)#spanning-tree mst configuration SW2(config-mst)#revision 1 SW2(config-mst)#name REGION1 SW2(config-mst)#instance 1 vlan 1,3,5,7,9 SW2(config-mst)#exit SW2(config)#spanning-tree mst 1 priority 16384 SW2(config)#end %SYS-5-CONFIG_I: Configured from console by console SW2#
Step 4. – Configure SW2 to run VLANs 2, 4, 6, 8 and 10 on MST instance 2 then configure SW2 as the root bridge for VLANs 2, 4, 6, 8 and 10 using the static priority of 8192.
SW2#configure terminal Enter configuration commands, one per line. End with CNTL/Z. SW2(config)#spanning-tree mst configuration SW2(config-mst)#instance 2 vlan 2,4,6,8,10 SW2(config-mst)#exit SW2(config)#spanning-tree instance 2 priority 8192 SW2(config)#end %SYS-5-CONFIG_I: Configured from console by console SW2#
Step 5. – Configure SW3 to run MST using the revision number 1 and region name REGION1 and assign VLANs 1,3,5,7,9 to instance 1, VLANs 2,4,6,8,10 to instance 2. Verify that the MST Root bridges for instance 1 and instance 2 are correct on SW3.
SW3 con0 is now available Press RETURN to get started. SW3>enable SW3#configure terminal Enter configuration commands, one per line. End with CNTL/Z. SW3(config)#spanning-tree mode mst SW3(config)#spanning-tree mst configuration SW3(config-mst)#revision 1 SW3(config-mst)#name REGION1 SW3(config-mst)#instance 1 vlan 1,3,5,7,9 SW3(config-mst)#instance 2 vlan 2,4,6,8,10 SW3(config-mst)#end SW3#
To verify that SW1 and SW2 are the root bridges for their respected VLANs by using the show spanning-tree mst # command for instance 1 and 2 as shown below;
SW3#show spanning-tree mst 1,2 ##### MST1 vlans mapped: 1,3,5,7,9 Bridge address 0014.a964.2e00 priority 32769 (32768 sysid 1) Root address 0014.f2d2.4180 priority 8193 (8192 sysid 1) port Fa0/10 cost 200000 rem hops 19 Interface Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- ----------------------------- Fa0/10 Root FWD 200000 128.10 P2p Fa0/15 Altn BLK 200000 128.15 P2p ##### MST2 vlans mapped: 2,4,6,8,10 Bridge address 0014.a964.2e00 priority 32770 (32768 sysid 2) Root address 001c.57d8.9000 priority 8194 (8192 sysid 2) port Fa0/15 cost 200000 rem hops 19 Interface Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- ----------------------------- Fa0/10 Altn BLK 200000 128.10 P2p Fa0/15 Root FWD 200000 128.15 P2p SW3#
As shown above you can see that MST1 (instance 1) has VLANs 1,3,5,7,9 mapped to it and the root bridge has a priority of 8193 and the MAC address of 0014.f2d2.4180. The root port is specified as Fa0/10 which is directly connected to SW1.
VLANs 2,4,6,8,10 are mapped to MST2 (Instance 2) and the root bridge has a priority of 8193 and the MAC address of 001c.57d8.9000. The root port specified for MST2 is Fa0/15 which is directly connected to SW2.
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