Multiple Spanning Tree Protocol (802.1s)

This protocol is inspired by Cisco proprietary Per-Vlan Spanning tree plus (PVST+). Cisco PVST+ creates an STP instance for each Vlan.

MSTP has more flexibility than PVST+, MSTP instances can map unlimited number of Vlans. MSTP supports 16 STP instances.

When MSTP is enabled, RSTP is essentially enabled as well.

The disadvantage of PVST+ is that if there are 1000 vlans, there will be 1000 STP instances. MSTP can have more flexibility in determining the number of STP instances; administrator can configure two STP instances with MSTP, each STP instance handles 500 Vlans for example.

MST Region

1. Switches within a region have the same configuration attributes.

2. Configuration attributes includes:

a. name

b. revision (this is the version of the MSTP update)

c. instances that map with respective vlans.

Example of MST configuration attributes:

3560-2(config)#spanning-tree mst configuration
3560-2(config-mst)#name mstp-region
3560-2(config-mst)#revision 1
3560-2(config-mst)#instance 1 vlan 100-300
3560-2(config-mst)#instance 2 vlan 400-600

Instance types

1. Internal Spanning Tree (IST) – One per region. Instance 0 is an IST. Treat IST as a management STP instance.

All vlans mapped to instance 0.

Instance 0 is responsible to provide loop free spanning tree network for all STP instances within a region. IST is the only instance in the region that sends and receives BPDU.

Modifying hello, max age and forward delay timer can only be done in Instance 0.

2. One or more Multiple Spanning Tree Instance (MSTI) within a region, Cisco supports maximum 16 instances (one IST + 15 MSTIs).

According to 802.1s, a MST enabled switch must support at least two STP instances – 1 IST + 1 MSTI.

All MSTIs within a region share the same STP timer (hello, max age and forward delay).

Each MSTI has its own STP topology which is mapped to vlans.

3. Common Spanning Tree (CST) – This instance operates as a first level of CIST. CST is formed by interconnection of MST regions. Each MST region is treated as a pseudo switch.

MST regions connect to one another through the connection of MST regional switch’s STP boundary port.

4. Common and Internal Spanning Tree (CIST) – This instance is the collection of various IST from MST regions. In other words, IST of each MSTP region when connected with loop is known as CIST.

Each MST region will have one IST (instance 0) that has the overall STP topology.

Very good reference:


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