IntroductionThis document describes VLAN frame format and QinQ packet encapsulation format. Show VLAN Frame FormatA switch identifies packets from different VLANs according to the information contained in its VLAN tags. IEEE 802.1Q adds a 4-byte VLAN tag between the Source/Destination MAC address and Length/Type fields of an Ethernet frame to identify the VLAN to which the frame belongs. Figure 1-1 shows the position of a VLAN tag in a VLAN data frame. Figure 1-1 IEEE 802.1Q frame format The following are the two types of Ethernet frames in a VLAN:
There are two main types of Ethernet links: access links (transmit untagged frames) and trunk links (transmit tagged frames). The two link types differ in the number of VLANs they can carry traffic for: an access link can carry traffic for only one VLAN, and therefore usually connects a switch to a user terminal, such as a host, a server, or an unmanaged switch; a trunk link can carry traffic for multiple VLANs, and as such usually connects a switch to another switch or a router. The frames transmitted on an access link are untagged frames, and the frames transmitted on a trunk link are tagged frames. All frames processed on a switch carry VLAN tags. After receiving an untagged frame from a user terminal, the switch adds a VLAN tag to the frame, recalculates the frame check sequence (FCS), and then transmits the frame through a trunk link. Before sending the frame to a user terminal, the switch removes the VLAN tag, and sends the untagged frame to the terminal through an access link. A VLAN tag contains four fields. Table 1 describes the fields. Table 1-1 Fields in a VLAN tag
QinQ Packet Encapsulation FormatHowever, because the 12-bit VLAN tag field defined in IEEE 802.1Q identifies a maximum of 4096 VLANs, the number of users that 802.1Q VLANs can identify and isolate on metro Ethernet networks (MANs) is insufficient. To address this issue, QinQ is developed to expand VLAN space beyond 4096 VLANs so that a larger number of users can be identified on MANs. QinQ is developed to expand VLAN space by adding an additional 802.1Q tag to an 802.1Q packet. This increases the number of VLANs to 4094 x 4094. The switch forwards packets over a public network according to the outer VLAN tag carried in the packets, and learns MAC addresses from the outer VLAN tag. The inner VLAN tag in the packets is forwarded as payload of the packets. QinQ is applied in scenarios other than simply to expand VLAN space. For example, because the inner and outer VLAN tags can be used to differentiate packets based on users and services, the inner tag can represent a user and the outer tag can represent a service. Figure 1-2 802.1Q encapsulation and QinQ encapsulation Frame Size The maximum frame length allowed by each interface on a carrier network should be at least 1504 bytes to accommodate the additional length of QinQ packets compared with 802.1Q packets. By default, the maximum frame length allowed on a switch is larger than 1504 bytes and so does not require modification. TPID Different carriers may use different TPID values in outer VLAN tags of QinQ frames. To ensure that QinQ frames sent from a Huawei device can be transmitted across a carrier network, set the TPID value to that used by the carrier. Table 1-2 provides some of the common protocol types and their TPID values, which should not be used in such cases. Table 1-2 Protocol types and values
What is the size of the VLAN tag in an Ethernet frame?The VLAN tag is a two-byte field inserted between the source MAC address and the Ethertype (or length) field in an Ethernet frame.
What is the maximum frame size for a VLAN based network?A VLAN is a local area network that utilises another LAN, rather than its own physical network, to carry data. The 802.1Q tagging protocol allows the Ethernet frame size to increase by four bytes to a range of 68 to 1522 bytes. This size increase is due to the insertion of a four-byte VLAN tag into the frame.
What is the maximum Ethernet frame size?To recap, Ethernet has a minimum frame size of 64 bytes, comprising an 18-byte header and a payload of 46 bytes. It also has a maximum frame size of 1518 bytes, in which case the payload is 1500 bytes.
What is the maximum size of a frame?Maximum Size Frame
The largest frame consists of 1526 Bytes (see table above) with a 9.6 µs inter-frame gap (corresponding to 12 Bytes at 10 Mbps). The total utilised period (measured in bits) therefore corresponds to1538 Bytes.
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If the vlan tag is present in an ethernet frame, what is the maximum frame size?
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