Per-Vlan Spanning Tree Groups; Using Multiple Stgs To Eliminate False Loops - Lenovo CN4093 Application Manual

Per-VLAN Spanning Tree Groups

Using Multiple STGs to Eliminate False Loops

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CN4093 Application Guide for N/OS 8.3
PVRST mode supports a maximum of 128 STGs, with each STG acting as an
independent, simultaneous instance of STP.
Multiple STGs provide multiple data paths which can be used for load-balancing
and redundancy. To enable load balancing between two CN4093s 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 2-128 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 CN4093 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 CN4093, 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
Application Switch
With a single Spanning Tree,
one link becomes blocked.
In the second network, the problem of improper link blocking is resolved when the
VLANs are placed into different Spanning Tree Groups (STGs). Since each STG has
its own independent instance of Spanning Tree, each STG is responsible only for
the loops within its own VLAN. This eliminates the false loop, and allows both
VLANs to forward packets between the switches at the same time.
STG 1
False
x
VLAN 1
Loop
is active
Using multiple STGs,
both links may be active.
Switch 2
STG 2
VLAN 30
is active
Application Switch