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CCNA IP Routing Practice Question

A router learns a route to 172.16.0.0/16 via OSPF (administrative distance 110) and a route to 172.16.10.0/24 via EIGRP (administrative distance 90). No other overlapping routes exist. Which TWO statements about how the router handles these routes are correct?

⚠ Common exam trap

Cisco often tests the misconception that administrative distance alone determines which route is installed, ignoring the critical role of prefix length in the longest prefix match rule.

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

✓

Both the OSPF and EIGRP routes are installed in the routing table.

B is correct because the router installs both routes in the routing table when they have different prefix lengths. The EIGRP route to 172.16.10.0/24 (AD 90) is more specific than the OSPF route to 172.16.0.0/16 (AD 110). The router uses the longest prefix match rule for forwarding, so both routes coexist without conflict.

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 router installs only the EIGRP route because it has a lower administrative distance.
Why it's wrong here
Administrative distance is only compared when two routes have the exact same prefix length and network address. Here, OSPF's 172.16.0.0/16 and EIGRP's 172.16.10.0/24 differ in prefix length, so they are treated as different destinations. The router installs both routes; the lower AD of EIGRP does not disqualify the OSPF route.
✓
Both the OSPF and EIGRP routes are installed in the routing table.
Why this is correct
Because 172.16.0.0/16 and 172.16.10.0/24 are not identical prefixes, the router treats them as separate destinations in the routing table. The RIB can hold a less-specific covering route and a more-specific route simultaneously, regardless of administrative distance. Longest-prefix matching during packet forwarding then determines which route is used for a given destination.
✗
Traffic to 172.16.10.100 is forwarded using the OSPF route.
Why it's wrong here
Packet forwarding uses longest-prefix match, not administrative distance. The address 172.16.10.100 falls inside the 172.16.10.0/24 subnet, so the EIGRP route with the 24-bit mask is the most specific match and is selected. The OSPF /16 route would only be considered if no longer prefix matched, but here the /24 always wins for addresses within its range.
✗
The EIGRP route is used for all traffic destined to any address within 172.16.0.0/16.
Why it's wrong here
The EIGRP route covers only the 172.16.10.0/24 subnet, not the entire 172.16.0.0/16 range. Traffic to addresses such as 172.16.11.1 or 172.16.20.1 does not match that /24 prefix, so those packets use the OSPF /16 route instead. The EIGRP route only affects destinations inside its specific /24 subnet, leaving the aggregate route active for everything else.
✓
The OSPF route is used for destinations within 172.16.0.0/16 that are not part of the 172.16.10.0/24 subnet.
Why this is correct
The OSPF 172.16.0.0/16 route is a covering route for the whole major network, but it is not used when the destination matches the more-specific 172.16.10.0/24. For any address within 172.16.0.0/16 that does not fall within the /24 subnet, the router finds only the OSPF /16 match, so that route is used. This is the standard behavior that allows a summary route to coexist with a more-specific child route.

Option-by-option analysis

Why each answer is right or wrong

Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.

✓Both the OSPF and EIGRP routes are installed in the routing table.Correct answer▾

Why this is correct

Because 172.16.0.0/16 and 172.16.10.0/24 are not identical prefixes, the router treats them as separate destinations in the routing table. The RIB can hold a less-specific covering route and a more-specific route simultaneously, regardless of administrative distance. Longest-prefix matching during packet forwarding then determines which route is used for a given destination.

✗The router installs only the EIGRP route because it has a lower administrative distance.Wrong answer — click to see why▾

Why this is wrong here

The router does not discard the OSPF route; it installs both /16 and /24 entries because they represent different network-specific entries.

✗Traffic to 172.16.10.100 is forwarded using the OSPF route.Wrong answer — click to see why▾

Why this is wrong here

The traffic matches the /24 route, not the /16, so it would be forwarded via the EIGRP next-hop.

✗The EIGRP route is used for all traffic destined to any address within 172.16.0.0/16.Wrong answer — click to see why▾

Why this is wrong here

The /24 is a subset; traffic outside 172.16.10.0/24 matches only the /16 OSPF route.

Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”

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.

Go deeper

CCNA IP Routing Practice Question

Why each option matters

Option-by-option breakdown

Why each answer is right or wrong

Routing Protocol Comparison

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