EIGRP Connectivity Issues During Peak Hours

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.