VLAN Tagging, also known as Frame Tagging, is a method developed by Cisco to help identify packets travelling through trunk links. When an Ethernet frame traverses a trunk link, a special VLAN tag is added to the frame and sent across the trunk link .How does ERS use the VLAN tag?（）

• A、provide service internetworking
• B、support transparency for Layer 2 frames
• C、indicate destination as a connection identifier
• D、map to the DLCI in service internetworking