The extension of a basic MAC frame is a virtual local area network (VLAN) tagged frame, which contains an additional 4-byte field for the VLAN tag and information between the source address and length/type fields. VLAN tagging is defined by the IEEE Standard 802.1Q. VLAN tagging can identify and separate many groups' network traffic from each other in enterprise and metro networks. Each VLAN group can consist of many users with varied MAC address in different geographical locations of a network. VLAN tagging increases and scales the network performance and add privacy and safety to various groups and customers' network traffic.
VLAN tagged frames have a maximum length of 1522 bytes, excluding the preamble and the SFD fields.
Figure 90. VLAN Tagged MAC Frame Format
In metro Ethernet applications, which require more scalability and security due to the sharing of an Ethernet link by many service providers, MAC frames can be tagged with two consecutive VLAN tags (stacked VLAN). Stacked VLAN frames contain an additional 8-byte field between the source address and client length/type fields, as illustrated.
Figure 91. Stacked VLAN Tagged MAC Frame Format