Q1: Is my understanding correct? [Port's transmit settings] U(Untagged): Send frames without VLAN ID T(Tagged): Send frames with VLAN ID.
[Port's receive setting] Acceptable Frame: Accept untagged frames or not. Ingress Filtering: When a tagged frame that is not a VLAN member is received, to which port should it be forwarded or discarded?
Q2: What is the purpose of Acceptable Frame and Ingress Filtering? In what cases do you allow filtering?
It defines if the port does accept tagged frames only (reject untagged frames), or if the port does accept tagged and untagged frames and does assign it to the [PVID-defined] VLAN on the switch.
According to the RT*M resp. the online help:
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In the Acceptable Frame Types list, specify how you want the port to handle untagged and priority tagged frames.
Whichever you select, VLAN tagged frames are forwarded in accordance with the IEEE 802.1Q VLAN standard. The default is Admit All.
VLAN Only. The port discards any untagged or priority tagged frames it receives.
Admit All. Untagged and priority tagged frames received on the port are accepted and assigned the value of the Port VLAN ID for this port.
Q2: ...and Ingress Filtering? In what cases do you allow filtering?
...in case you want frames tagged on VLAN IDs not defined on the port to the switch, e.g. to prohibit incoming frames with VLAN tags probably used elsewhere in the network.
According to the RT*M resp. the online help:
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In the Ingress Filtering list, specify how you want the port to handle tagged frames. You have the following options:
Enable. A tagged frame is discarded if this port is not a member of the VLAN identified by the VLAN ID in the tag. In an untagged frame, the VLAN is the Port VLAN ID specified for the port that received this frame.
Disable. All frames are forwarded in accordance with the IEEE 802.1Q VLAN standard. The default is Enable.
