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APRIL 2008
MSTP INTEROPERABILITY OF THE DELL™ POWERCONNECT™ 6200 SERIES SWITCHES
WITH CISCO IOS AND CISCO CATOS-BASED SWITCHES
2. Ensuring that each FID supported by the Bridge is allocated to exactly one
Spanning Tree Instance.
The Dell PowerConnect 62xx series switch implements this through the FID to MSTI
Allocation Table. The following diagram shows an example configuration:
The combination of VID to FID and then FID to MSTI allocation defines a mapping of VIDs to
spanning tree instances, represented by the MST Configuration Table. The following diagram
shows an example configuration:
Figure 2: Example FID to MSTI Allocation
Figure 3: Example Resultant VID to MSTI Allocation
With this allocation, we ensure that every VLAN is assigned to one and only one MSTI. The
CIST is also an instance of spanning tree with an MSTID of 0. We can have an instance which
has no VIDs allocated to it but every VLAN must be allocated to one of the other instances of
spanning tree.
The portion of the active topology of the network that connects any two bridges in the same
MST region traverses only MST bridges and LANs in that region and never bridges of any
kind outside the region. In other words, connectivity within the region is independent of
external connectivity.
ACTIVE TOPOLOGY ENFORCEMENT
Each received frame is allocated to a spanning tree instance by the forwarding process using
the VID. The forwarding process selects each port as a potential transmission port if, and only
if all of the following conditions are met:
1. The port on which the frame was received is in forwarding mode for that spanning tree instance.
2. The port considered for transmission is in a forwarding state for that spanning tree instance.
3. The port considered for transmission is not the same port on which the frame was received.
For each port not selected as a potential transmission port, the frame is discarded.