STP: Multiple Spanning-Tree

MST is a standard published by the IEEE to create different STP instances for groups of VLANs to have more than one STP topology approaching the Cisco mechanisms that we have seen previously. We opted for vlan groups and not for one instance of each vlan to not give too much workload to the switch.

MST is different from 802.1Q and PVST+, although it can interoperate with them. If a switch is configured to use MST, it must somehow figure out which of its neighbors are using MST as well. This is done by configuring switches in MST regions, where each switch in a region runs MST with compatible parameters.

In most networks, a single MST region is sufficient, although you can configure more than one region. Within the region, all switches must run the MST instance defined by the following attributes:

• MST configuration name (32 characters)
• MST configuration revision number (0 to 65535)
• MST instance to VLAN mapping table (4096 entries)

When the switches have the same attributes configured, they will be in the same region. If the attributes are not the same, the switch is considered to be in the boundary of the region. It can connect to another MST region but also communicate with a switch running another version of STP.

The MST revision name and number can be chosen by us as long as it is configured on all switches in the same region. The VLANs will be assigned to an instance via the MST instance to the VLAN assignment table, this is something we have to do ourselves.

Within the MST region, we will have an instance of STP that will create a loop-free topology within the region. When configuring MST, a default instance is always used to calculate the topology within the region. The instance is called IST (Internal Spanning Tree). By default, Cisco will use instance 0 to run IST.

All VLANs are mapped to instance 0 (IST) by default. The IST is used to communicate with other switches that do not speak MST; the IST is coupled to the STP that is running on the neighboring switch; it can be STP or RSTP. What it does is that the switch that speaks MST looks like a single very large switch that encompasses all the switches in the STP region. And that giant switch speaks the STP or RSTP protocol through the IST or instance 0.

According to the IEEE 802.1s specification, a switch with MST must be able to handle at least these two instances:

• Internal Spanning-Tree (IST)
• One or more instances of Multiple Spanning-Tree (MSTI)

The Cisco implementation supports 16 instances: one IST (instance 0) and 15 MSTIs.

Let’s see how to configure MST on Cisco switches.

Step 1. Enable MST on the switch:

Switch(config)# spanning-tree mode mst

Step 2. Enter the MST configuration mode:

Switch(config)# spanning-tree mst configuration

Step 3. Assigns a configuration name to the region (up to 32 characters):

Switch(config-mst)# name  <region_name>

Step 4.Assign a revision number for the region (0 to 65,535):

Switch(config-mst)# revision <revision_number>

The configuration revision number provides you with a means of tracking changes to the MST region configuration. Each time you make changes to the configuration, you must increment the number by one. Remember that the region configuration (including the revision number) must match on all switches in the region. Therefore, you also need to update the revision numbers on the other switches to match.

Step 5. Assign VLANs to an MST instance:

Switch(config-mst)# instance <instance_id> vlan <vlan_list>

Remember that the vlans are assigned by default to instance 0 (IST).

Step 6. Display the pending changes you have made:

Switch(config-mst)# show pending

Step 7. Exit the MST configuration mode; to activate the mode:

Switch(config-mst)# exit

A switch cannot run MST and PVST+ at the same time.

The default MST costs change a bit according to the port speeds, the new costs are:

• 10  Gbps – 2000
• 1   Gbps – 20000
• 100 Mbps – 200000

The above commands are to configure MST, the MSTI instances, and the vlan declarations in each instance. Now let’s see that there are general commands in MST that we have discussed in all versions of STP, such as changing bridge priority, port costs, port priority, timers, etc.

Switch(config)# spanning-tree mst instance-id root {primary | secondary} {diameter diameter} 
Switch(config)# spanning-tree mst instance-id priority bridge-priority 
Switch(config)# spanning-tree mst instance-id cost cost 
Switch(config)# spanning-tree mst instance-id port-priority port-priority 
Switch(config)# spanning-tree mst hello-time seconds 
Switch(config)# spanning-tree mst forward-time seconds Switch(config)# spanning-tree mst max-age seconds

Let’s see an example of MST configuration :

Switch(config)# vlan 1-5 
Switch(config)# spanning-tree mode mst 
Switch(config)# spanning-tree mst configuration 
Switch(config-mst)# name Jmcristobal 
Switch(config-mst)# revision 1 
Switch(config-mst)# instance 1 vlan 1-10,80 
Switch(config-mst)# instance 2 vlan 100-150

Verification commands:

Switch# show spanning-tree mst configuration 
Switch# show spanning-tree //Solo muestra la instacia 0 - IST

For more information:

https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/24248-147.html