Which Cisco proprietary protocol aggregates Ethernet ports on network devices?

• A. Port Aggregation Protocol (PAGP)
• B. Link Aggregation Control Protocol (LACP)
• C. Spanning Tree Protocol (STP)
• D. Virtual Local Area Network (VLAN)

Answer: (A)

Port aggregation, or EtherChannel, combines several physical Ethernet links into one logical link to increase bandwidth and provide redundancy. Cisco devices use Port Aggregation Protocol (PAGP) to negotiate and manage this aggregation. PAGP is Cisco proprietary, meaning it’s specific to Cisco gear and used to automatically form and maintain the EtherChannel when both ends support it. It handles which links participate in the channel and ensures both sides agree on the active members, giving a single, resilient path for traffic.

In contrast, Link Aggregation Control Protocol (LACP) is the IEEE standard (widely supported across vendors) for similar link-aggregation functionality, so it’s not Cisco-specific. Spanning Tree Protocol focuses on preventing loops in a network topology and does not aggregate links. VLANs segment networks at layer 2 and do not bind multiple physical ports into a single logical link.