IEEE 802.1Q Frame
IEEE 802.1Q uses an internal tagging mechanism which inserts a 4-byte tag field in the original Ethernet frame itself between the Source Address and Type/Length fields. Because the frame is altered, the trunking device recomputes the FCS on the modified frame.
| DA | SA | TAG | TYPE/LEN | DATA | FCS |
This example shows the further expansion of the Tag field. The expansion includes the field acronyms and the number of bits for each field.
| No. of bits | 16 | 3 | 1 | 12 |
| Frame field | TPID | PRIORITY | CFI | VID |
Field Descriptions
This section provides detailed descriptions of the 802.1Q frame fields.TPID—Tag Protocol Identifier
The Tag Protocol Identifier is a 16-bit field. It is set to a value of 0x8100 in order to identify the frame as an IEEE 802.1Q-tagged frame.Priority
Also known as user priority, this 3-bit field refers to the IEEE 802.1p priority. The field indicates the frame priority level which can be used for the prioritization of traffic. The field can represent 8 levels (0 through 7).CFI—Canonical Format Indicator
The Canonical Format Indicator is a 1-bit field. If the value of this field is 1, the MAC address is in noncanonical format. If the value is 0, the MAC address is in canonical format.VID—VLAN Identifier
The VLAN Identifier is a 12-bit field. It uniquely identifies the VLAN to which the frame belongs. The field can have a value between 0 and 4095.Frame Size
The 802.1Q tag is 4 bytes. Therefore, the resulting Ethernet frame can be as large as 1522 bytes. The minimum size of the Ethernet frame with 802.1Q tagging is 68 bytes.QinQ
The QinQ Support feature adds another layer of IEEE 802.1Q tag (called "metro tag" or "PE-VLAN") to the 802.1Q tagged packets that enter the network. The purpose is to expand the VLAN space by tagging the tagged packets, thus producing a "double-tagged" frame. The expanded VLAN space allows the service provider to provide certain services, such as Internet access on specific VLANs for specific customers, yet still allows the service provider to provide other types of services for their other customers on other VLANs.When a tunnel port receives tagged customer traffic from an 802.1Q trunk port, it does not strip the received 802.1Q tag from the frame header; instead, the tunnel port leaves the 802.1Q tag intact, adds a 2-byte Ethertype field (0x8100) followed by a 2-byte field containing the priority (CoS) and the VLAN. The received customer traffic is then put into the VLAN to which the tunnel port is assigned. This Ethertype 0x8100 traffic, with the received 802.1Q tag intact, is called tunnel traffic.
Frame Size
The default maximum transmission unit (MTU) of an interface is 1500 bytes. With an outer VLAN tag attached to an Ethernet frame, the packet size increases by 4 bytes. Therefore, it is advisable that you appropriately increase the MTU of each interface on the provider network. The recommended minimum MTU is 1504 bytes.TPID
The QinQ frame contains the modified tag protocol identifier (TPID) value of VLAN Tags. By default, the VLAN tag uses the TPID field to identify the protocol type of the tag. The value of this field, as defined in IEEE 802.1Q, is 0x8100.The device determines whether a received frame carries a service provider VLAN tag or a customer VLAN tag by checking the corresponding TPID value. After receiving a frame, the device compares the compares the configured TPID value with the value of the TPID field in the frame. If the two match, the frame carries the corresponding VLAN tag. For example, if a frame carries VLAN tags with the TPID values of 0x9100 and 0x8100, respectively, while the configured TPID value of the service provider VLAN tag is 0x9100 and that of the VLAN tag for a customer network is 0x8200, the device considers that the frame carries only the service provider VLAN tag but not the customer VLAN tag.
In addition, the systems of different vendors might set the TPID of the outer VLAN tag of QinQ frames to different values. For compatibility with these systems, you can modify the TPID value so that the QinQ frames, when sent to the public network, carry the TPID value identical to the value of a particular vendor to allow interoperability with the devices of that vendor. The TPID in an Ethernet frame has the same position with the protocol type field in a frame without a VLAN tag. In order to avoid problems in packet forwarding and handling in the network, you cannot set the TPID value to any of the values in this table:
| Protocol type | Value |
| ARP | 0x0806 |
| PUP | 0x0200 |
| RARP | 0x8035 |
| IP | 0x0800 |
| IPv6 | 0x86DD |
| PPPoE | 0x8863/0x8864 |
| MPLS | 0x8847/0x8848 |
| IS-IS | 0x8000 |
| LACP | 0x8809 |
| 802.1x | 0x888E |
The QinQ Support feature is generally supported on whatever Cisco IOS features or protocols are supported. For example, if you can run PPPoE on the subinterface, you can configure a double-tagged frame for PPPoE. IPoQinQ supports IP packets that are double-tagged for QinQ VLAN tag termination by forwarding IP traffic with the double-tagged (also known as stacked) 802.1Q headers.
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