5-10
Multiple Instance 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 5-3 each instance has a
different forwarding path.
Figure 5-3. 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.
Region “X”
Switch 1
IST Root
VLAN Memberships:
• IST Instance: VLANs 1, 2
• MSTI “A”: 4, 5
• MSTI “B”: 7, 9
Switch 2
MSTI “A” Root
VLAN Memberships:
• IST Instance: VLANs 1, 2
• MSTI “A”: 4, 5
• MSTI “B”: 7, 9
Switch 3
MSTI “B” Root
VLAN Memberships:
• IST Instance: VLANs 1, 2
• MSTI “A”: 4, 5
• MSTI “B”: 7, 9
Path through IST Instance
to Other Regions
Blocks redundant
link for MSTI “B”.
Blocks redundant
link for MSTI “A”.
Blocks redundant
link for IST instance.
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