31.1.3 About coexistence with various spanning tree

(1) Coexistence with PVST+

For PVST+, if only one VLAN is set for the VLAN mapping of the Ring Protocol, the VLAN can be used with the Ring Protocol. When the axrp virtual-link configuration command is used to set a virtual link, topologies are built by virtual links, and usage with the Ring Protocol starts.

Under the initial Ring Protocol configuration settings, all running PVST+ instances are stopped, and then started sequentially for VLANs for which a VLAN mapping is set. If multiple VLANs are set for a VLAN mapping, PVST+ will not run for the VLANs. Note that loops might occur for VLANs for which PVST+ is stopped. Perform port blockage or other actions to prevent loop configurations.

Because virtual links cannot be built when the axrp virtual-link configuration command has not been used to set a virtual link, the intended topology cannot be built, which might cause loops to occur.

The figure below shows a configuration in which PVST+ and the Ring Protocol are used together. In the figure, because only one VLAN 30 is set for VLAN mapping 128, it runs as PVST+. Because multiple VLANs are set for VLAN mapping 1, PVST+ cannot run. Also, because VLAN 100 is set as the virtual link VLAN for switches C and D, a virtual link is built between both switches.

Figure 31-4 Coexistence of PVST+ and Ring Protocol

(2) Coexistence with Single Spanning Tree

Single Spanning Tree can be used with all data VLANs for which the Ring Protocol is running.

For Single Spanning Tree, when the axrp virtual-link configuration command is used to set a virtual link, a topology based on the virtual link is built and usage with the Ring Protocol starts. When the axrp virtual-link configuration command is not used to set a virtual link, the intended topology cannot be built because virtual links cannot be built. As a result, loops might occur.

The figure below shows a configuration in which Single Spanning Tree and the Ring Protocol are used together. In the figure, Single Spanning Tree is set for switches C, D, and G, and two VLAN groups for the Ring Protocol are set for switches A, B, C, D, E, and F. Each topology for Single Spanning Tree is applied to the VLANs belonging to all VLAN groups (all VLAN mappings). Also, because VLAN 100 is set as the virtual link VLAN for switches C and D, a virtual link is built between both switches.

Figure 31-5: Configuration of single spanning tree and Ring Protocol

(3) About Simultaneous Operation of PVST+ and Single Spanning Tree

Even when used with the Ring Protocol, PVST+ and Single Spanning Tree can be used at the same time. In this case, all VLANs not running with PVST+ are run as Single Spanning Tree (the same as for normal concurrent operation).

The figure below shows a configuration in which Single Spanning Tree, PVST+, and the Ring Protocol are used together. In the figure, because only one VLAN 30 is set for VLAN mapping 128, it runs as PVST+. Because PVST+ is not running for VLAN mapping 1, it runs as Single Spanning Tree, and reflects the topology. Also, because VLAN 100 is set as the virtual link VLAN for switches C and D, a virtual link is built between both switches.

Figure 31-6: Configuration of single spanning tree, PVST +, and Ring Protocol

(4) Coexistence with Multiple Spanning Tree

Multiple Spanning Tree can be used with all VLANs for data transfer for which the Ring Protocol is running.

For Multiple Spanning Tree, when the axrp virtual-link configuration command is used to set a virtual link, a topology based on the virtual link is built and usage with the Ring Protocol starts. When the axrp virtual-link configuration command is not used to set a virtual link, the intended topology cannot be built because virtual links cannot be built. As a result, loops might occur.

When the same VLAN is set for the VLAN belonging to the MST instance and the Ring Protocol VLAN mapping, it can be run together for both the MST instance and the Ring Protocol. If the set VLANs do not match, the unmatched VLAN is put in Blocking status.

The figure below shows a configuration in which Multiple Spanning Tree and the Ring Protocol are used together. In the figure, Multiple Spanning Tree is set for switches C, D, and G, and two VLAN groups for the Ring Protocol are set for switches A, B, C, D, E, and F. The topology is reflected to Multiple Spanning Tree with VLAN group 1 of the Ring Protocol as CIST and VLAN group 2 as MST instance 3. Also, because VLAN 100 is set as the virtual link VLAN for switches C and D, a virtual link is built between both switches.

Figure 31-7: Configuration of Multiple Spanning Tree and Ring Protocol

(5) VLAN that do not coexist and operate

• VLANs with only the Ring Protocol applied

  When PVST+ is stopped by, for example, configuration settings, the VLAN only has the Ring Protocol applied.

  During Single Spanning Tree operation or Multiple Spanning Tree operation, VLANs for data transfer handled by the Ring Protocol must run together.

• VLANs that have only PVST+ applied

  When a VLAN mapping not belonging to a VLAN group is set for the Ring Protocol, the VLAN has only PVST+ applied.

• VLANs that have only Single Spanning Tree applied

  VLANs that do not belong to a VLAN group for the Ring Protocol have only Single Spanning Tree applied.

• VLANs that have only Multiple Spanning Tree applied

  VLANs that do not belong to a VLAN group for the Ring Protocol have only Multiple Spanning Tree applied.