Quick Answer
The answer is that the 10.0.0.0/8 route via Null0 is a discard route created to prevent routing loops. When EIGRP is configured with the summary-address command on an interface, it automatically installs a summary route pointing to Null0 on the router that generated the summary. This ensures that if a packet matches the summary but not any of the more specific component routes (like 10.0.0.0/24 or 10.0.1.0/24), it is dropped rather than forwarded back upstream, which could cause a loop. On the Cisco CCNP ENARSI 300-410 exam, this concept tests your understanding of EIGRP route summarization and loop prevention mechanisms. A common trap is confusing this Null0 route with a static discard route or thinking it represents an actual path; in reality, it is automatically generated and has an administrative distance of 5. Memory tip: think of the Null0 summary as a "black hole" that catches any traffic that misses the more specific routes, keeping the routing table loop-free.
This 300-410 practice question tests your understanding of route summarization. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A network engineer runs the following command on Router R1:
R1# show ip eigrp topology summary
IP-EIGRP Topology Table for AS(100)/ID(1.1.1.1) Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply, r - reply Status, s - sia Status
P 10.0.0.0/8, 1 successors, FD is 2812416, serno 10 via Summary (2812416/0), Null0 P 10.0.0.0/24, 1 successors, FD is 2172416, serno 5 via 192.168.1.2 (2172416/2812416), GigabitEthernet0/0 P 10.0.1.0/24, 1 successors, FD is 2172416, serno 6 via 192.168.1.2 (2172416/2812416), GigabitEthernet0/0
Based on this output, what is the purpose of the route 10.0.0.0/8 via Null0?
It is a default route for unknown destinations.
Why wrong: It is a summary route, not a default route.
It is a discard route to prevent routing loops.
The Null0 interface indicates this is a discard route, commonly used with summarization to avoid loops.
It is a route learned from a neighbor.
Why wrong: It is via Summary, not from a neighbor.
It is a connected route.
Why wrong: It is not connected; it is a summary route.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
It is a discard route to prevent routing loops.
The route 10.0.0.0/8 via Null0 is a summary route created by the 'summary-address' command in EIGRP, used to prevent routing loops by discarding packets that do not match more specific routes.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
It is a default route for unknown destinations.
Why it's wrong here
It is a summary route, not a default route.
It is a discard route to prevent routing loops.
Why this is correct
The Null0 interface indicates this is a discard route, commonly used with summarization to avoid loops.
Related concept
OSPF neighbours must agree on key parameters.
It is a route learned from a neighbor.
Why it's wrong here
It is via Summary, not from a neighbor.
It is a connected route.
Why it's wrong here
It is not connected; it is a summary route.
Common exam traps
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Real-world example
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
What to study next
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
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FAQ
Route Summarization — This question tests Route Summarization — OSPF neighbours must agree on key parameters..
The correct answer is: It is a discard route to prevent routing loops. — The route 10.0.0.0/8 via Null0 is a summary route created by the 'summary-address' command in EIGRP, used to prevent routing loops by discarding packets that do not match more specific routes.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
OSPF neighbours must agree on key parameters.
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Same concept, more angles
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A network engineer runs the following command on Router R1: R1# show ip eigrp topology 10.0.0.0 255.255.252.0 IP-EIGRP (AS 100): Topology entry for 10.0.0.0/22 State: Passive, Origin: Internal, Metric [90/2172416], Tag 0 Number of successors: 1 FD is 2172416, Serno: 5 Route is Summary Advertised by R2 (via Serial0/0/0) Reply status: 0 Based on this output, what is true about the route 10.0.0.0/22?
mediumWhy B: The 'Route is Summary' line indicates this is a summary route in the EIGRP topology table. The output shows it is passive and has a successor, confirming it is a valid summary route.
Variation 2. A network engineer runs the following command on Router R1: R1# show ip eigrp interfaces detail GigabitEthernet0/0 IP-EIGRP interfaces for process 100 Interface Peers Xmit Queue Mean Pacing Time Multicast Pending Un/Reliable SRTT Un/Reliable Flow Timer Routes Gi0/0 1 0/0 10 0/10 50 0 Hello interval: 5 sec, Hold time: 15 sec Split horizon is enabled Summary address: 10.0.0.0/8 Next xmit serial <none> Un/reliable mcasts: 0/0 Un/reliable ucasts: 0/0 Mcast exceptions: 0 CR packets: 0 ACKs suppressed: 0 Retransmissions: 0 Retry timer: 15 Hello packets sent: 100, received: 99 Based on this output, what is the purpose of the summary address configured on this interface?
mediumWhy B: The 'Summary address: 10.0.0.0/8' line shows that a manual summary route is configured on this interface, which will be advertised to EIGRP neighbors.
Variation 3. A network engineer runs the following command on Router R1: R1# show ip route 10.0.0.0 255.255.252.0 longer-prefixes Routing entry for 10.0.0.0/22 Known via "eigrp 100", distance 90, metric 2172416, type internal Last update from 192.168.1.2 on GigabitEthernet0/0, 00:00:10 ago Routing Descriptor Blocks: * 192.168.1.2, from 192.168.1.2, 00:00:10 ago, via GigabitEthernet0/0 Route metric is 2172416, traffic share count is 1 Routing entry for 10.0.1.0/24 Known via "eigrp 100", distance 90, metric 2812416, type internal Last update from 192.168.1.2 on GigabitEthernet0/0, 00:00:10 ago Routing Descriptor Blocks: * 192.168.1.2, from 192.168.1.2, 00:00:10 ago, via GigabitEthernet0/0 Route metric is 2812416, traffic share count is 1 Based on this output, what is the effect of the summary route 10.0.0.0/22?
hardWhy B: The presence of both the summary route (10.0.0.0/22) and a more specific route (10.0.1.0/24) in the routing table indicates that the summary route is not suppressing the more specific routes, possibly due to a configuration issue or because the summary is not configured as a discard route.
Last reviewed: Jun 18, 2026
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