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Understanding STGs and VLANs

For the purposes of Spanning Tree Protocol negotiation, the ports on a 8000 Series switch can be divided into groups of ports where each group of ports performs its own spanning tree negotiation with neighboring devices. In a 8000 Series switch, these groups of ports are called spanning tree groups (STGs).

The ports in a VLAN are always a subset of the ports in a STG. A VLAN can include all the ports in a given STG, and there can be multiple VLANs in a STG, but a VLAN will never have more ports than exist in the STG. Because VLANs are always subsets of STGs, the recommended practice is to plan STGs and then create VLANs.

In the default configuration, the 8000 Series switch contains a single STG encompassing all the ports in the switch. For most applications, this configuration is sufficient. The default STG has ID 1 (STG1).

If a VLAN spans multiple switches, it must be within the same STG across all switches; that is, the ID of the STG in which it is defined must be the same across all devices.

See also:


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