Reference Guide

Multiple Spanning-Tree Protocol
Multiple Spanning-Tree Protocol (MSTP) is an RSTP-based spanning-tree variation that improves on per-VLAN RPVST+. You
can configure Multiple Spanning-Tree Instances (MSTIs) and map multiple VLANs to one spanning-tree instance to reduce the
total number of required instances. RPVST+ allows a spanning-tree instance for each VLAN. This 1:1 approach is not suitable if
you have multiple VLANs — each spanning-tree instance costs bandwidth and processing resources.
When you enable MST, all ports in Layer 2 mode participate in MST. Keep in mind that OS10 only supports one MST region.
Load balancing can be achieved using the MSTP. When three VLANs are mapped to two MSTIs, VLAN 100 traffic takes a
different path than VLAN 200 and 300 traffic.
Configuring MST is a four-step process:
1. Enable MST, if the current running STP version is not MST.
2. (Optional) Map the VLANs to different instances to achieve load balancing.
3. Ensure the same region name is configured in all the bridges running MST.
4. (Optional) Configure the revision number.
Configure MSTP
When you enable MST globally, all L2 physical, port-channel, and VLAN interfaces are automatically assigned to MSTI zero (0).
Within an MSTI, only one path from any one bridge to another is enabled for forwarding.
● Enable MST in CONFIGURATION mode.
spanning-tree mode mst
Configure and verify MSTP
OS10(config)# spanning-tree mode mst
OS10(config)# do show spanning-tree
show spanning-tree mst configuration
Region Name: ravi
Revision: 0
MSTI VID
0 1,7-4093
1 2
2 3
3 4
4 5
5 6
Add or remove interfaces
By default, all interfaces are enabled in L2 switchport mode, and all L2 interfaces are part of spanning-tree.
196
Layer 2