In a simple topology of a single switch with two Access ports configured, for example fa0/1 on VLAN 10 and fa0/2 on VLAN 20, how does a switch know not to send broadcasts between these vlans if they are access ports and thus untagged. As they are unchanged Ethernet frames sent by hosts on these VLANs until they are tagged by a trunk port, how does a switch differentiate them internally?