Multiple Spanning Protocol is IEEE 802.1w standard protocol which provides multiple forwarding path for data traffic. It provides the load balancing and also reduces the number of spanning tree instance in STP topology.
It reduces the number of spanning tree instance because it maps more than two VLANS in a spanning tree instance. MST is not default STP mode rather RSTP is default STP mode in Nexus 7000 Series Switches.
MST instance with same region name, revision number and same VLAN to instance mapping form a MST region. From outside MST region looks like a single bridge. As multiple VLANs are mapped to single instance, MST produces one BPDU per instance and due to this feature, number of BPDU decreases considerably in layer 2 domain unlike RSTP where it produces one BPDU per VLAN.
MST also have backward interoperability with previous version of STP.
- 802.1D STP
- Rapid PVST+
When a set of devices configured with same MST configuration forms the MST region. This configuration includes same region name, revision number and same VLAN to instance mapping. Member of region must be capable of processing the 802.1w BPDU.
As per today, there is no limit of MST region in network however each device can only support 65 instance only including instance 0 in a single MST region.
Instance are identified by any numerical number between 1 to 4094 and a VLAN can be a member of one instance at time.
To another MST region, RSTP or 802.1D STP, MST region appears as single bridge to them.
When MST is configured, MST generates the one BPDU per instance and that BPDU contains M records for each MSTI on the device. This MST BPDU carries information about instance and its associated VLANs in it.
MST Configuration information:
When MST is configured, the configuration should be same who want to become member of same region.
While configuring MST configuration, the three parameters needs to be configured:
- Name: Name mentioned will identify MST region
- Revision Number: Identify revision number of current MST Configuration, But it will increment automatically even MST configuration are changes number of times.
- VLAN to MST Instance Mapping: Each instance can be mapped to unique VLAN and a VLAN cannot be part of two instance. Instance number varies between 1 to 4096 for VLAN 1 to 4096. Any VLAN not mapped to any instance will automatically become part of instance 0.
A MST region will accept the MST BPDU if the information provided will match with its own MST configuration. If any configuration parameters differs MST bridge treats the BPDU from any another MST region.
IST, CIST, and CST Overview
IST is referred as Instance 0 in which all VLAN are mapped by default. This IST cannot be deleted and this is only STP which receives and send BPDU.
In each region, MST establish additional spanning trees and these spanning tree are called as MSTIs. These MSTIs information is contained in MST records called M-records which are further encapsulated with in MST BPDU. Each MSTI in a region has different topology parameters like root-bridge, STP path cost , root path cost etc. All MSTIs in a region are local to that region and is independent to MSTIs from another region.
CIST is the collection of multiple IST in each MST region.
CST is a spanning tree instance which interconnects MST region along with 802.1D and 802.1w instance running on network. CST encompasses all MST regions along with 802.1D instance and behaves like 1 single STP instance for entire bridge network.
MST interoperability with 802.1D:
When any MST switch receives the BPDU with Protocol version 0, it migrates itself to 802.1D switch and process that BPDU. A MST device with treat that port where it receives the 802.1D BPDU as border port However if that devices stops receiving 802.1D BPDU, it will not revert back to MST mode.
So in order to restart the protocol migration process you must clear the STP with following commands:
Clear spanning-tree detected-protocols