What can be inferred from the configuration line 'SW1(config-if)#vlan trunk allow 1,20'?

The configuration line 'SW1(config-if)#vlan trunk allow 1,20' indicates that two VLANs, VLAN-1 and VLAN-20, are permitted on the trunk link. In networking, the concept of a native VLAN is crucial for understanding how VLAN-tagged frames interact with untagged frames on a trunk port.

In this scenario, VLAN-1 is commonly used as the default native VLAN on many networking devices, including those from Aruba. When a trunk link is configured, untagged traffic that comes to that link is typically associated with the native VLAN. Since the provided command includes VLAN-1, it infers that VLAN-1 serves as the native VLAN for this trunk setting.

Therefore, while the configuration permits both VLAN-1 and VLAN-20, the assertion that the native VLAN is VLAN-1 is justified as it aligns with industry standards where VLAN-1 is the default unless specifically changed. Hence, the inference that the native VLAN is VLAN-1 is correct in the context of this configuration.