Q-in-Q Ethernet Frame Fields

A

IEEE 802.1ad is an Ethernet networking standard informally known as QinQ and is an amendment to IEEE standard IEEE 802.1Q-1998.

The conventions for 802.1ad terminology typically are as follows:

The inner tag is the tag which is closest to the payload portion of the frame; it is officially called C-TAG (Customer tag, with ethertype 0x8100).

The outer tag is the one closer/closest to the Ethernet header; its name is S-TAG (Service tag, ethertype 0x88a8).

Q-in-Q (also known as VLAN stacking) is a technique used to add additional VLAN tags to an Ethernet frame to enable multiple VLANs to be transported over a single Ethernet link. This is commonly used in service provider networks to provide layer 2 VPN services to customers.

When using Q-in-Q, two VLAN tags are added to the Ethernet frame, with the outer VLAN tag identifying the customer's VLAN and the inner VLAN tag identifying the service provider's VLAN. This allows the service provider to maintain separation between different customer VLANs and to provide layer 2 connectivity between customer sites.

The answer to the question is A. S-VLAN and C-VLAN. The S-VLAN (Service Provider VLAN) is the outer VLAN tag that identifies the service provider's VLAN, while the C-VLAN (Customer VLAN) is the inner VLAN tag that identifies the customer's VLAN.

So, when an Ethernet frame is sent using Q-in-Q, it is first tagged with the C-VLAN tag, then the S-VLAN tag is added to the front of the frame. The S-VLAN tag includes information about the service provider's network, such as the VLAN ID, priority, and EtherType. The C-VLAN tag includes information about the customer's network, such as the VLAN ID, priority, and EtherType.

In summary, Q-in-Q adds two VLAN tags to the Ethernet frame: the outer S-VLAN tag and the inner C-VLAN tag. The S-VLAN tag identifies the service provider's VLAN, while the C-VLAN tag identifies the customer's VLAN. Therefore, the correct answer is A. S-VLAN and C-VLAN.