Which three statements about EIGRP route summarization are correct? (Choose three.)
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A. B. C. D. E.BCE.
EIGRP (Enhanced Interior Gateway Routing Protocol) is an advanced distance-vector routing protocol used in computer networks for IP routing. Route summarization is a process used to summarize a group of IP routes as a single route, which reduces the number of routing entries in the routing table and minimizes the traffic on the network.
The correct statements about EIGRP route summarization are as follows:
A. By default, all EIGRP routes are automatically summarized; all routes will be summarized unless the no-auto summary command is configured. This statement is incorrect. By default, EIGRP does not summarize routes. To enable route summarization, you need to configure it manually or by using the auto-summary command. The no-auto summary command disables route summarization.
B. By default, EIGRP automatically summarizes internal routes, but only each time a major network boundary is crossed. This statement is partially correct. By default, EIGRP does not summarize routes, including internal routes. If you enable route summarization, it can be done manually or by using the auto-summary command. When summarization is enabled, EIGRP can summarize internal routes at major network boundaries, but not automatically.
C. EIGRP route summarization can reduce the query diameter to help prevent SIA problems. This statement is correct. EIGRP route summarization can reduce the query diameter, which is the maximum number of routers that EIGRP will search for a path to a destination. By reducing the query diameter, route summarization can prevent SIA (Stuck-In-Active) problems. SIA occurs when a router is waiting for a reply from its neighbor, but the neighbor has not responded due to a routing loop.
D. Summary routes are inserted in the routing table with a next hop of null 0 and a high administrative distance, to prevent black holing of traffic. This statement is correct. When a summary route is created, it is inserted into the routing table with a next hop of null 0 and a high administrative distance (by default, 5), which means that the summary route cannot be used for forwarding packets. This prevents black holing of traffic.
E. The metric for each summarized route is inherited from the lowest metric of the component routes. This statement is correct. When a summary route is created, the metric for the summary route is inherited from the lowest metric of the component routes. This ensures that the summary route is selected as the best path to the destination.
In summary, statements C, D, and E are correct about EIGRP route summarization. Statements A and B are incorrect, and statement B is partially correct.