SPRUIU1C July 2020 – February 2024 DRA821U , DRA821U-Q1
Untagged packets are all packets that are not a VLAN packet or a priority tagged packet. According to the CPSW_ALE_UVLAN_UNTAG[1-0] UVLAN_FORCE_UNTAGGED_EGRESS bit in the packet header the packet may exit the switch with a VLAN tag inserted or the packet may leave the switch unchanged. The two cases are discussed below.
Untagged input packets have the header packet VLAN inserted when the CPSW_ALE_UVLAN_UNTAG[1-0] UVLAN_FORCE_UNTAGGED_EGRESS bit in the transmit packet header is de-asserted. For untagged packets, the VLAN EtherType = 0x8100 is inserted after the source address followed by the two byte header packet VLAN. The header packet VLAN is composed of the header packet priority along with the PORT_CFI and and PORT_VID values from the CPSW_PN_PORT_VLAN_REG_k register (where N is the port that the untagged packet entered the switch) through. The packet length/type field is output four bytes later than it is input and is not removed or replaced. If the CRC is present in the packet data (PASS_CRC is asserted), then the packet CRC is replaced with a hardware generated CRC.
Untagged input packets are output unchanged when the CPSW_ALE_UVLAN_UNTAG[1-0] UVLAN_FORCE_UNTAGGED_EGRESS transmit packet header bit is asserted.