194 G8264 Application Guide for ENOS 8.4Per-VLAN Spanning Tree GroupsPVRST mode supports a maximum of 256 STGs, with each STG acting as anindependent, simultaneous instance of STP. STG 256 can only be used formanagement traffic.Notes: By default, a maximum number of 128 STGs is supported. To upgrade the valueto 256, configure the switch accordingly and reboot. Make sure the PVRSTswitches are both booted with 128 STGs profile or 256 STGs profile. For optimal performance, it is recommended to stay within a maximum of 10interfaces or aggregated links (including vLAGs) with 4K MAC table entrieswhen deploying 256 STP groups.Multiple STGs provide multiple data paths which can be used for load‐balancingand redundancy. To enable load balancing between two G8264s using multipleSTGs, configure each path with a different VLAN and then assign each VLAN to aseparate STG. Since each STG is independent, they each send their own IEEE802.1Q tagged Bridge Protocol Data Units (BPDUs).Each STG behaves as a bridge group and forms a loop‐free topology. The defaultSTG 1 may contain multiple VLANs (typically until they can be assigned toanother STG). STGs 2‐256 may contain only one VLAN each.Using Multiple STGs to Eliminate False LoopsFigure 13 shows a simple example of why multiple STGs are needed. In the figure,two ports on a G8264 are connected to two ports on an application switch. Each ofthe links is configured for a different VLAN, preventing a network loop. However,in the first network, since a single instance of Spanning Tree is running on all theports of the G8264, a physical loop is assumed to exist, and one of the VLANs isblocked, impacting connectivity even though no actual loop exists.Figure 13. Using Multiple Instances of Spanning Tree GroupVLAN 1VLAN 30Application SwitchSwitch 1FalseLoopApplication SwitchSwitch 2VLAN 1is activeSTG 1VLAN 30is activeSTG 2xWith a single Spanning Tree,one link becomes blocked.Using multiple STGs,both links may be active.
