Spanning-Tree Operation

802.1s Multiple Spanning Tree Protocol (MSTP)

How Separate Instances Affect MSTP Operation. Assigning different groups of VLANs to different instances ensures that those VLAN groups use independent forwarding paths. For example, in figure 5-26 each instance has a different forwarding path.

Path through IST Instance to Other Regions

Region “X

Switch 1

IST Root

VLAN Memberships:

• IST Instance: VLANs 1, 2

• MSTI “A”: 4, 5

• MSTI “B”: 7, 9

 

 

Blocks redundant

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Blocks redundant

 

 

 

link for MSTI “B”.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

link for MSTI “A”.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Switch 2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Switch 3

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

MSTI “A” Root

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

MSTI “B” Root

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

VLAN Memberships:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

VLAN Memberships:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Blocks redundant

 

 

 

 

• IST Instance: VLANs 1, 2

 

 

 

 

 

 

 

 

• IST Instance: VLANs 1, 2

 

 

 

 

 

 

link for IST instance.

 

 

 

 

• MSTI “A”: 4, 5

 

 

 

 

 

 

 

 

• MSTI “A”: 4, 5

• MSTI “B”: 7, 9

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

• MSTI “B”: 7, 9

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 5-26. Active Topologies Built by Three Independent MST Instances

While allowing only one active path through a given instance, MSTP retains any redundant physical paths in the instance to serve as backups (blocked) paths in case the existing active path fails. Thus, if an active path in an instance fails, MSTP automatically activates (unblocks) an available backup to serve as the new active path through the instance for as long as the original active path is down. Note also that a given port may simultaneously operate in different states (forwarding or blocking) for different spanning-tree instances within the same region. This depends on the VLAN memberships to which the port is assigned. For example, if a port belongs to VLAN 1 in the IST instance of a region and also belongs to VLAN 4 in MSTI “x” in the same region, the port may apply different states to traffic for these two different instances.

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