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Hy vọng rằng, những câu hỏi này sẽ giúp hạn chế số lượng câu hỏi thi mà bạn thu hẹp lựa chọn của bạn hai lựa chọn và sau đó đoán. Bạn có thể tìm thấy câu trả lời cho những câu hỏi tại Phụ lục A. | 156 Chapter 6 VLANs and Trunks If an edge switch receives such a frame on its tunnel port should it blindly encapsulate the frame into the tunnel or should it try to process the frame itself as an important control message from another neighboring switch Control protocol PDUs STP VTP CDP are normally sent over VLAN 1 on a trunk. When these protocols are received at a service provider s tunnel port they are interpreted by the edge switch rather than being tunneled. STP and VTP are dropped not accepted because they don t directly apply to the service provider s internal network. The CDP frames however are interpreted because the edge switch thinks it should learn of its connected neighbors. The net result is that none of these protocols are forwarded on across the tunnel as the customer expects. To remedy this a Layer 2 Protocol Tunnel can be used at the service provider edge that performs Generic Bridge PDU Tunneling GBPT . Here the edge switch receives these frames from the customer s trunk and rewrites them to have a GBPT destination MAC address of a Cisco proprietary multicast address . The encapsulated frames are then sent into the tunnel as if they came from the native VLAN on the customer s trunk. Other switches in the provider s network recognize the GBPT destination address and unencapsulate the control PDUs. GBPT can be performed on the control protocols selectively so only the desirable protocols are tunneled. Configuring Layer 2 Protocol Tunneling To configure Layer 2 Protocol tunneling use the following commands Switch config interface type mod port Switch config-if l2protocol-tunnel cdp stp vtp Switch config-if l2protocol-tunnel drop-threshold pps cdp stp vtp Switch config-if l2protocol-tunnel shutdown-threshold pps cdp stp vtp This feature must be configured on every service provider edge switch so that the control protocols can be encapsulated and unencapsulated correctly. In the first l2protocol-tunnel .