Can an interface be both tagged and untagged

Well the general rule around untagged vs tagged interface is this:

Untagged = 1 VLAN per interface

Tagged = 802.1q tagging (multiple VLANS on an interface), or a TRUNKED interface in Cisco terms.

However, it is possible to configure an interface as both Tagged and Untagged. This is actually something that is not that uncommon. The VLAN tag is just a header that the network device will match on. If it matches, then it will continue the processing of the packet up the OSI layers. To give you some examples:

Interface have the following configuration:

Tagged:

• VLAN10
• VLAN20

Untagged:

• VLAN30

1. When traffic arrives on the intermediary device (for instance a switch) with either VLAN10 or VLAN20, the traffic will leave the switch with the VLAN tag of VLAN10 or VLAN20. Since the traffic has the VLAN header assigned, the BIG-IP will try and match the VLAN tag with the ones configured on the Tagged Interface. If it does not match, it will drop the traffic. It it matches it will continue the processing up the OSI layers.

2. When traffic arrives on the intermediary device with VLAN30 which is untagged on the switch, the traffic will leave the switch with no VLAN tag assigned. Since the traffic does not have any VLAN tag assigned, it will be automatically accepted by the BIG-IP since it does not expect a VLAN tag. The traffic will continue the processing up the OSI layers.

I hope this helps. Let me know if you have any further questions.