Which of the following protocols can be used to negotiate EtherChannel configurations between Cisco switches? (Select all that apply.)
Correct Answer: A,B
Detailed Explanation: EtherChannel is a technology that enables the bundling of multiple physical links into a single logical channel for increased bandwidth and redundancy. To automatically negotiate and maintain these aggregated links, certain protocols are used between the devices that form the EtherChannel. The two primary protocols used for this purpose are Port Aggregation Protocol (PAgP) and Link Aggregation Control Protocol (LACP). A. Port Aggregation Protocol (PAgP): PAgP is a Cisco proprietary protocol designed to facilitate the automatic creation of EtherChannel links between Cisco devices. PAgP actively negotiates the channel formation, ensuring that both sides of the link agree to bundle the links together. PAgP can operate in either an “auto” mode (passively waiting for negotiation) or a “desirable” mode (actively initiating negotiation). B. Link Aggregation Control Protocol (LACP): LACP is an IEEE-standard protocol (defined in IEEE 802.3ad, now part of IEEE 802.1AX) that performs a similar function as PAgP. It allows devices from different vendors to interoperate when forming aggregated links. LACP dynamically detects the configuration of the physical links and negotiates the formation of the EtherChannel. Its standardized nature makes LACP a widely accepted solution in mixed-vendor environments. C. Spanning Tree Protocol (STP) (Incorrect): STP is used to prevent network loops in a Layer 2 network by blocking redundant paths. It does not play a role in negotiating link aggregation; instead, it operates independently to ensure loop-free topologies. While STP runs on all switch ports—including those participating in EtherChannel—its function is entirely separate from EtherChannel negotiation. D. Cisco Discovery Protocol (CDP) (Incorrect): CDP is a Cisco proprietary protocol used for device discovery and gathering information about directly connected Cisco devices. It is not involved in the negotiation or configuration of EtherChannel links. Real-World Impact: Using PAgP or LACP for EtherChannel negotiation enhances network resilience. These protocols ensure that the links are bundled only when both sides agree on the configuration, reducing the risk of misconfiguration. Moreover, if one or more links in the EtherChannel fail, the protocol can adjust and continue to provide connectivity over the remaining links. This dynamic adjustment is crucial for maintaining high availability in enterprise networks. Conclusion: For negotiating EtherChannel configurations between Cisco switches, the correct protocols are PAgP and LACP (Options A and B). These protocols enable automatic link bundling, providing increased bandwidth, redundancy, and load balancing. Understanding which protocols serve this purpose—and knowing that neither STP nor CDP are used for this function—is essential for effective network design and troubleshooting.
This CCNA practice question helps students prepare for Cisco networking certification exams by testing knowledge of network fundamentals, routing, switching, and network security concepts.