A network engineer connects two Catalyst switches with a single link and expects an 802.1Q trunk to form dynamically. On both switches the interface is configured with 'switchport mode dynamic auto' and no other switchport commands. After the link comes up, VLAN traffic from non-native VLANs is not passing between the switches. 'show interfaces switchport' on both sides shows the operational mode as 'static access'. What is the cause of this behavior?