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How do I get RSTP on my ATI switch to work with PVST+ ?


PVST+ (Per-VLAN Spanning Tree) is a version of Spanning Tree developed and implemented by Cisco, though some vendors such as HP and Brocade implement it as well.  ATI switches, both AlliedWare and AlliedWare Plus operating systems, implement RSTP as the default spanning tree version--and do NOT implement PVST+.  As such, there can be problems with the proper operation of spanning tree when connecting ATI switches into a mixed-vendor topology.

PVST+ and rapid-PVST+ send out two types of BPDU's, IEEE standard and SSTP (shared spanning tree protocol).  SSTP is part of and exclusive to PVST+ operation.  Generating additional standard BPDU's allows a PVST+ switch to interop with non-PVST+ switches running STP, RSTP and MSTP.  However, PVST+ switches communicate IEEE standard BPDU's exclusively via untagged 802.1Q frames on VLAN 1, even if a different native VLAN VID is explicitly defined.

Therefore, to maintain proper spanning tree operation, ATI recommends observing these rules when connecting to PVST+ switches:
  • connections between switches must be 802.1Q trunk links
  • the native VLAN or PVID should be VLAN 1 on the ATI and PVST+ switches
  • VLAN 1 must be allowed untagged on all trunk links
A more detailed explanation and configuration examples can be reviewed on the attached  How To Guide.

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