Per-Vlan Spanning Tree Groups; Using Multiple Stgs To Eliminate False Loops - Lenovo RackSwitch G8264 Application Manual

Per-VLAN Spanning Tree Groups

Using Multiple STGs to Eliminate False Loops

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G8264 Application Guide for ENOS 8.4
PVRST mode supports a maximum of 256 STGs, with each STG acting as an
independent, simultaneous instance of STP. STG 256 can only be used for
management traffic.
Notes:
 By default, a maximum number of 128 STGs is supported. To upgrade the value
to 256, configure the switch accordingly and reboot. Make sure the PVRST
switches are both booted with 128 STGs profile or 256 STGs profile.
 For optimal performance, it is recommended to stay within a maximum of 10
interfaces or aggregated links (including vLAGs) with 4K MAC table entries
when deploying 256 STP groups.
Multiple STGs provide multiple data paths which can be used for load‐balancing
and redundancy. To enable load balancing between two G8264s using multiple
STGs, configure each path with a different VLAN and then assign each VLAN to a
separate STG. Since each STG is independent, they each send their own IEEE
802.1Q tagged Bridge Protocol Data Units (BPDUs).
Each STG behaves as a bridge group and forms a loop‐free topology. The default
STG 1 may contain multiple VLANs (typically until they can be assigned to
another STG). STGs 2‐256 may contain only one VLAN each.
Figure 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 of
the 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 the
ports of the G8264, a physical loop is assumed to exist, and one of the VLANs is
blocked, impacting connectivity even though no actual loop exists.
Figure 13. Using Multiple Instances of Spanning Tree Group
Switch 1
False
x
Loop
Application Switch
With a single Spanning Tree,
one link becomes blocked.
Switch 2
STG 1
VLAN 1
is active
Application Switch
Using multiple STGs,
both links may be active.
STG 2
VLAN 30
is active