Mismatched Trunking Operational States
Trunking can be configured correctly so that both switches forward frames for the same set of VLANs. However, trunks can also be misconfigured, with a couple of different results. In some cases, both switches conclude that their interfaces do not trunk. In other cases, one switch believes that its interface is correctly trunking, while the other switch does not.
The most common incorrect configuration—which results in both switches not trunking—is a configuration that uses the switchport mode dynamic auto command on both switches on the link. The word “auto” just makes us all want to think that the link would trunk automatically, but this command is both automatic and passive. As a result, both switches ...
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