EIGRP Connectivity Issues During Peak Hours
Refer to the exhibit. An engineer notices a connectivity problem between routers R1 and R2. The frequency of this problem is high during peak business hours. Which action resolves the issue?
Community Insight
The problem tests the understanding that EIGRP requires a matched MTU for adjacency formation, and large packets dropped due to MTU issues cause intermittent connectivity.
This question addresses EIGRP neighbor adjacency failures occurring during high traffic periods, specifically focusing on the impact of MTU mismatches on routing protocol stability.
Many users incorrectly select Option A (Increase bandwidth) because they associate 'peak hours' with congestion, ignoring that EIGRP neighbors must establish adjacency before data flows are affected by bandwidth limits.
Community Discussion (3 comments)
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Expert Analysis
Why the Answer Is Correct
The correct action is to ensure the MTU is consistent across the link between R1 and R2. EIGRP uses multicast packets (Hello packets and Updates) to establish and maintain neighbor relationships. If an interface has a lower MTU than its neighbor, large EIGRP Update packets may be fragmented or dropped if the Don't Fragment (DF) bit is set (which it often is in modern implementations to avoid fragmentation overhead). This results in the loss of Hello packets or successful Updates but failed Acknowledgments, leading to flapping adjacencies or complete failure to form neighbors. While peak hours increase traffic volume, the root cause of the connectivity problem (adjacency instability) is the MTU mismatch preventing reliable control plane communication.Why the Other Options Are Wrong
Option A is incorrect because increasing bandwidth does not fix a Layer 3 MTU mismatch; even with high bandwidth, packets exceeding the path's minimum MTU will still be dropped if DF is set. Option B is dangerous; decreasing keepalive timers would make the link more sensitive to transient packet loss, potentially causing more frequent neighbor resets during peak hours rather than resolving the issue. Option D is incorrect because static neighbor configuration does not bypass the underlying physical/link layer constraint of MTU; if the MTU is mismatched, the static neighbors will still fail to exchange necessary updates or hellos properly.Community Comment Notes
One user pointed out that "EIGRP DUAL-3-SIA" errors are often linked to missing or incorrect bandwidth configurations, which can influence path selection and lead to Stuck-In-Active states. However, another commenter strongly argued for Option C, noting that MTU mismatches are a classic cause of such intermittent failures. The consensus leans towards addressing the packet size limitation as the primary suspect for adjacency drops under load.Official Reference
Exam Strategy
When troubleshooting routing protocol issues that seem related to traffic volume, always check Layer 3 parameters like MTU first. High traffic doesn't just mean congestion; it means more packets being sent, which increases the likelihood of hitting MTU limits if they are mismatched.
Frequently Asked Questions
Why doesn't increasing bandwidth fix EIGRP issues?
Bandwidth affects throughput, not packet size. An MTU mismatch causes packet drops regardless of available bandwidth, breaking neighbor adjacencies.
Can EIGRP work with mismatched MTUs?
Generally no. EIGRP updates may exceed the smaller MTU and be dropped if the DF bit is set, causing neighbor timeouts and flapping.
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