Which of the following is true about the bridging mechanism used for PPP tunnels?

• A. Bridging control protocol
• B. Bridge protocol
• C. Spanning tree protocol
• D. Encapsulation protocol

Answer: (A)

To carry Ethernet frames over a PPP link, you need a control mechanism that manages bridging between the two ends of the tunnel. The Bridging Control Protocol is the PPP-level mechanism designed for this purpose: it negotiates and maintains the ability for the PPP peers to act as a bridge, allowing Ethernet frames to be forwarded across the tunnel as if both ends were on the same LAN. This is what enables MAC learning and proper forward/filter behavior over the PPP connection. The other options don’t fit because a generic Bridge Protocol isn’t a PPP control mechanism, Spanning Tree Protocol is used inside bridged networks to prevent loops (not to establish PPP bridging), and Encapsulation Protocol only defines how data is placed into PPP frames, not how bridging is negotiated or managed across the tunnel.