Question 42 of 2,152

Route Summarization →mediumMultiple ChoiceObjective-mapped

RIP Route Summarization Missing Subnet Troubleshooting

This 300-410 practice question tests your understanding of route summarization. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 is troubleshooting a route summarization issue in a network using RIP. Router R1 is configured with the 'ip summary-address rip 192.168.0.0 255.255.252.0' command on its serial interface. After the configuration, R2, which is connected via that interface, can no longer reach the 192.168.1.0/24 subnet, although other subnets within the summary are reachable. What is the most likely cause?

Clue words in this question

Noticing these words before you look at the options changes how you read each choice.

Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

A
The 192.168.1.0/24 subnet is not directly connected to R1, so it cannot be summarized.
Why wrong: Incorrect. RIP can summarize routes learned from other interfaces as well.
B
The summary route 192.168.0.0/22 is being advertised with a higher metric than the specific routes, causing R2 to prefer a different path.
Why wrong: Incorrect. The summary route would replace the specific routes, not compete with them.
C
The summary address command was applied on the wrong interface, causing the summary to be sent out all interfaces, including the one facing the 192.168.1.0/24 subnet's origin.
Why wrong: Incorrect. The summary is applied on a specific interface; it does not affect other interfaces.
D
The 192.168.1.0/24 subnet is not included in the summary range because the summary mask is /22, but the subnet's network address is 192.168.1.0, which is within the range, but the RIP process may have a split-horizon issue preventing the route from being advertised.
Correct. In RIP, split horizon prevents a route from being advertised out the interface it was learned on. If the 192.168.1.0/24 subnet was learned on the same interface where the summary is applied, it will not be advertised, causing loss of connectivity.

Full breakdown with real-world context →

Quick Answer

The answer is a split-horizon issue caused by the RIP summary route being generated on the same interface from which the specific 192.168.1.0/24 route was learned. When you configure the `ip summary-address rip 192.168.1.0 255.255.252.0` command, RIP suppresses the advertisement of more specific subnets within that /22 range, but split-horizon prevents the summary from being sent back out the interface where the specific route was originally received. This creates a scenario where R2 loses the 192.168.1.0/24 route because the summary is blocked, yet the specific route is also suppressed. On the Cisco CCNP ENARSI 300-410 exam, this tests your understanding of how RIP route summarization interacts with split-horizon, a common trap where engineers assume the summary automatically covers all subnets. The key memory tip: “Summarize, but don’t split—if the source is the same interface, the route won’t hit.”

Answer choices

Why each option matters

Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.

Correct answer & explanation

✓

The 192.168.1.0/24 subnet is not included in the summary range because the summary mask is /22, but the subnet's network address is 192.168.1.0, which is within the range, but the RIP process may have a split-horizon issue preventing the route from being advertised.

In RIP, split-horizon prevents a route learned on an interface from being advertised back out that same interface. If R1 learned the 192.168.1.0/24 route via the serial interface (e.g., from R2 or another neighbor), split-horizon would block R1 from including that route in the summary advertisement sent back out the serial interface. Consequently, R2 does not receive a route for 192.168.1.0/24, either as a specific route or as part of the summary, causing unreachability. Other subnets within the summary range that were not learned on that interface are correctly advertised, so they remain reachable.

Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

✗
The 192.168.1.0/24 subnet is not directly connected to R1, so it cannot be summarized.
Why it's wrong here
Incorrect. RIP can summarize routes learned from other interfaces as well.
✗
The summary route 192.168.0.0/22 is being advertised with a higher metric than the specific routes, causing R2 to prefer a different path.
Why it's wrong here
Incorrect. The summary route would replace the specific routes, not compete with them.
✗
The summary address command was applied on the wrong interface, causing the summary to be sent out all interfaces, including the one facing the 192.168.1.0/24 subnet's origin.
Why it's wrong here
Incorrect. The summary is applied on a specific interface; it does not affect other interfaces.
✓
The 192.168.1.0/24 subnet is not included in the summary range because the summary mask is /22, but the subnet's network address is 192.168.1.0, which is within the range, but the RIP process may have a split-horizon issue preventing the route from being advertised.
Why this is correct
Correct. In RIP, split horizon prevents a route from being advertised out the interface it was learned on. If the 192.168.1.0/24 subnet was learned on the same interface where the summary is applied, it will not be advertised, causing loss of connectivity.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.

Common exam traps

Common exam trap: OSPF can fail even when IP connectivity looks correct

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

How to think about this question

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.

KKey Concepts to Remember

OSPF neighbours must agree on key parameters.
Router ID selection can affect neighbour relationships and LSDB output.
OSPF cost influences the preferred path.
A route can appear in OSPF information but not become the installed route.

TExam Day Tips

Check area mismatch first when OSPF adjacency fails.
Review passive interfaces when a network is advertised but no neighbour forms.
Use show ip ospf neighbor and show ip route clues carefully.

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

How this comes up in practice

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.

Visual reference

Quick reference

Routing Protocol Comparison

Protocol	Metric	Max Hops	Algorithm	Type
RIP v2	Hop count	15	Bellman-Ford	Distance vector
OSPF	Cost (bandwidth)	Unlimited	Dijkstra (SPF)	Link state
EIGRP	Composite metric	Unlimited	DUAL	Hybrid
IS-IS	Cost	Unlimited	Dijkstra	Link state
BGP	Policy / attributes	Unlimited	Path vector	Path vector

RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.

What to study next

Got this wrong? Here's your next step.

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

Questions learners often ask

What does this 300-410 question test?

Route Summarization — This question tests Route Summarization — OSPF neighbours must agree on key parameters..

What is the correct answer to this question?

The correct answer is: The 192.168.1.0/24 subnet is not included in the summary range because the summary mask is /22, but the subnet's network address is 192.168.1.0, which is within the range, but the RIP process may have a split-horizon issue preventing the route from being advertised. — In RIP, split-horizon prevents a route learned on an interface from being advertised back out that same interface. If R1 learned the 192.168.1.0/24 route via the serial interface (e.g., from R2 or another neighbor), split-horizon would block R1 from including that route in the summary advertisement sent back out the serial interface. Consequently, R2 does not receive a route for 192.168.1.0/24, either as a specific route or as part of the summary, causing unreachability. Other subnets within the summary range that were not learned on that interface are correctly advertised, so they remain reachable.

What should I do if I get this 300-410 question wrong?

Are there clue words in this question I should notice?

Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

What is the key concept behind this question?

OSPF neighbours must agree on key parameters.

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