When a switch receives an untagged frame on a trunk port, which VLAN does it assume the frame belongs to?

When a switch receives an untagged frame on a trunk port, it assumes that the frame belongs to the native VLAN. The concept of the native VLAN is crucial in the context of VLAN tagging. The native VLAN is the VLAN that is used for untagged traffic on trunk links, allowing for the proper handling of frames that do not carry VLAN information.

In most configurations, VLAN 1 is the default native VLAN on Cisco switches unless it has been explicitly changed to a different VLAN. This means that any frame arriving on a trunk port without a VLAN tag will be associated with the designated native VLAN, ensuring that it can be processed correctly.

It's important to recognize that the choice of native VLAN can impact network security and configuration. If different switches on the same network have different native VLAN configurations, it can lead to potential issues such as VLAN hopping attacks or misrouting of packets. Thus, understanding the role of the native VLAN in handling untagged frames is essential for effective switch configuration and network management.