300-440 Question 27
Single answerA network engineer has configured EIGRP on multiple routers in an enterprise network. The engineer notices that when auto-summarization is enabled, some subnets are not reachable across the network. After disabling auto-summarization, reachability is restored. What is the most likely cause of this issue?
- A
Overlapping subnets from different major networks caused a routing conflict.
- B
The bandwidth of the links between routers was insufficient for summarization updates.
- C
EIGRP does not support manual summarization when auto-summarization is enabled.
- D
The auto-summarization feature caused EIGRP to advertise incorrect default routes.
Show answer and explanation
Correct answer: A
Explanation
When auto-summarization is enabled in EIGRP, the protocol summarizes routes at major network boundaries. If there are overlapping subnets in different major networks, this can lead to routing conflicts and loss of reachability. Disabling auto-summarization allows EIGRP to advertise the specific subnets instead of the summarized major network, resolving the issue. Understanding the impact of auto-summarization and its interaction with the network design is key to troubleshooting this type of problem.
- A. Correct.
This is correct. Auto-summarization causes EIGRP to advertise only the major network instead of the specific subnets. If overlapping subnets exist across different major networks, it can result in routing conflicts and loss of reachability.
- B. Incorrect.
This is incorrect. Bandwidth is not a factor in how EIGRP performs summarization. Summarization issues are related to routing logic, not link capacity.
- C. Incorrect.
This is incorrect. EIGRP supports manual summarization even when auto-summarization is enabled. The problem in this scenario is due to the behavior of auto-summarization, not the inability to configure manual summarization.
- D. Incorrect.
This is incorrect. Auto-summarization does not advertise incorrect default routes. It simply summarizes subnets into their major networks, which can cause reachability issues if not configured properly.