检查两边的trunk配置看看是否一致。估计是DELL 3548交换机不支持DTP,使得交换机之间不能够进行trunk协商。在cisco 3750交换机与Dell 3548交换机相连的接口上执行
Switch(config-if)# switchport nonegotiate
将接口设置为nonegotiate,不发送DTP帧,如果配置为非协商,那么就必须手工配置接口模式,为access或trunk
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Rockyw | If it solves your problem, please mark as answer. Thanks !