EtherChannel bundles multiple physical links into a single logical port-channel. The CCNA tests LACP vs PAgP vs static EtherChannel, the mode combinations that form a channel, and common configuration errors. These appear as configuration tasks and troubleshooting scenarios.
Start Scenario PracticeAn engineer is troubleshooting intermittent connectivity issues between two data center switches. The link is a 10GE LACP port-channel. Which misconfiguration could cause packet loss?
Explanation: Option D is correct because LACP requires one side to be in active mode to initiate negotiation; if one side is active and the other is passive, the passive side will not initiate the LACP exchange, but it will respond to active-side messages. However, the question states that the link is an LACP port-channel, implying both sides should be configured to form the bundle. If one side is passive and the other is active, the port-channel can form, but intermittent packet loss can occur if the passive side fails to respond quickly enough to LACP PDUs during transient conditions, or if there is a mismatch in LACP system priority or port priority that causes the bundle to flap. More critically, a passive/passive combination would never form the port-channel, but active/passive can form it, yet the passive side's reliance on the active side for initiation can lead to instability under certain failure scenarios, causing packet loss.
A network engineer is configuring EtherChannel between two Cisco Catalyst switches. The ports are configured as access ports in VLAN 10. After configuring the port-channel interface and adding the physical ports, the engineer notices that the EtherChannel does not come up. The show etherchannel summary command shows the port-channel in a down state. What is the most likely cause?
Explanation: The correct answer is that the physical ports must be configured identically, including the allowed VLAN list. For access ports, the VLAN must match. The wrong answers involve issues that would not prevent the channel from forming if the VLANs match.
A network engineer is troubleshooting an EtherChannel between two Cisco switches. The show etherchannel 1 port-channel command shows the port-channel is up, but traffic is not load-balanced evenly. The engineer notices that all traffic is using only one link. The physical ports are all configured identically. What is the most likely cause?
Explanation: The correct answer is that the load-balancing method is set to src-dst-ip, but the traffic is from a single source to a single destination IP. The wrong answers involve issues that would prevent the channel from being up or affect all links.
An engineer is troubleshooting an EtherChannel between two switches. The show etherchannel summary output shows that the port-channel is up, but only one physical link is active. The other three links are in a suspended state. The physical ports are all configured identically with the same VLAN allowed. What is the most likely cause?
Explanation: The correct answer is that the maximum number of active ports in the port-channel has been configured to 1. The wrong answers involve misconfigurations that would affect all ports or prevent the channel from forming.
A network engineer is configuring EtherChannel between two Cisco switches using LACP. The engineer wants to ensure that if fewer than two links are operational, the EtherChannel does not come up. Which command should be configured?
Explanation: The correct answer is 'port-channel min-links 2', which sets the minimum number of active links required. The wrong answers either set maximum links or are unrelated.
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Practice all EtherChannel and LACP ScenariosEtherChannel bundles multiple physical links into a single logical port-channel. The CCNA tests LACP vs PAgP vs static EtherChannel, the mode combinations that form a channel, and common configuration errors. These appear as configuration tasks and troubleshooting scenarios. These appear throughout the 350-401 and require you to apply your knowledge, not just recall facts.
Cisco doesn't publish an exact breakdown, but scenario-based questions (especially exhibit and command-output formats) make up a significant portion of the 350-401. Practicing each scenario type ensures you're ready for any format.
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