CCNA IP Routing Practice Test 25 — 15 Questions

Question 1

A network engineer configures OSPF on R1 and R2 over a point-to-point link. The interfaces are in the same OSPF area, with matching hello and dead timers, and are on the same IP subnet. However, the command show ip ospf neighbor on both routers shows no neighbors. A firewall sits between R1 and R2. What should the technician do next?

Accepted Answer

Check the firewall rules to verify that multicast address 224.0.0.5 is not being blocked.. OSPF uses multicast address 224.0.0.5 for hello packets on all OSPF network types except non-broadcast. With a firewall in the path, the most common reason for a complete lack of neighbor discovery, even when the interface-level OSPF parameters match, is that the firewall is blocking this multicast traffic. Checking the firewall rules directly addresses the most likely layer-3/layer-4 filtering issue and is a non-intrusive first step before examining deeper OSPF-specific settings.

Answer

Verify that the OSPF router IDs on R1 and R2 are unique.

Answer

Check the OSPF network type on the connecting interfaces.

Answer

Verify that the MTU on the connecting interfaces matches.

Question 2

A network engineer adds a loopback interface Lo0 with IP address 172.16.0.1/32 to router R1. After restarting the OSPF process, the OSPF router-ID changes from 10.1.1.1 to 172.16.0.1, and the neighbor relationship with R2 resets. What should the technician do next to prevent this disruption the next time a loopback is added?

Accepted Answer

Configure a static router-id using the router-id command under the OSPF process. Configuring a static router-id under the OSPF process ensures the router-ID never changes due to interface IP changes or loopback additions. This directly addresses the root cause of the adjacency reset—dynamic router-ID re-election.

Answer

Configure passive-interface Lo0 under the OSPF process

Answer

Set a higher OSPF priority on the router’s interfaces

Answer

Configure the OSPF area as a stub area

Question 3

Refer to the exhibit. A network engineer notices that traffic from R1 to the 10.1.0.0/16 network is taking a longer path than expected despite OSPF being the only routing protocol. The engineer examines the OSPF LSDB on R1 to investigate. Based on the output, what is the most likely cause of the suboptimal routing?

Accepted Answer

The ABR is configured with the 'area 1 range 10.1.0.0 255.255.0.0 cost 1000' command.. The exhibit shows three Type 3 summary LSAs from the same ABR (10.1.1.1). Two of them have metrics 10 and 20, which are consistent with normal OSPF intra-area path costs. The LSA for 10.1.0.0/16 has metric 1000, an abnormally high value. This indicates that the ABR is injecting this summary with a manually configured cost, typically done with the 'area range cost ' command. Global reference bandwidth changes would affect all metrics equally, and route filtering does not alter metric values. External route metric types are unrelated to Type 3 inter-area summaries.

Answer

The reference bandwidth has been misconfigured on R1, causing the OSPF cost calculation to be inflated for some links.

Answer

The ABR is filtering the 10.1.0.0/16 route from Area 1 into Area 0, causing the router to recalculate the metric higher.

Answer

The metric-type for OSPF external routes has been set to type 2, causing the metric to be inflated to 1000 for the 10.1.0.0/16 prefix.

Question 4

A technician is troubleshooting an OSPF network. On a broadcast segment, R1 is the DR and R2 is the BDR. R1's interface GigabitEthernet0/0 is shut down for maintenance. The technician expects that R2 will assume the DR role, but instead a new DR election occurs and another router is elected DR. What is the most likely cause?

Accepted Answer

R2 has an OSPF priority of 0.. When the DR fails on a broadcast OSPF network, the BDR normally takes over as DR and a new BDR is elected. However, if the BDR's OSPF priority is set to 0, the router is ineligible to become DR or BDR. With the DR down and the BDR ineligible, the remaining routers must hold a fresh election, and the router with the highest non-zero priority (or highest router-ID if priorities tie) becomes the new DR. Therefore, R2's priority of 0 explains why it did not become DR and a new election was triggered.

Answer

The OSPF hello and dead intervals on R2 do not match those of other routers on the segment.

Answer

R2 has an OSPF priority of 255.

Answer

The OSPF network type on the segment was changed to point-to-point.

Question 5

After configuring a static discard route with the command 'ip route 0.0.0.0 0.0.0.0 Null0 254' on R1, a network engineer finds that traffic to destinations that are not in the routing table is still being forwarded out to the ISP instead of being dropped. What is the most likely cause?

Accepted Answer

The ISP static default route has an administrative distance of 1, which is lower than the administrative distance of 254 set for the Null0 route.. The static default route via the ISP has an administrative distance of 1 (either explicitly configured or inherited as a static route), which is lower than the Null0 route's AD of 254. Because the router always prefers the route with the lowest administrative distance for the same destination, the ISP route is installed in the forwarding table, and the discard route is never used. For the discard route to be selected, its AD would need to be less than or equal to the ISP route's AD, or the ISP route must be removed.

Answer

The Null0 interface is administratively shut down.

Answer

The address 0.0.0.0/0 is an invalid destination for a static route because it is not a real network.

Answer

The static discard route must include a next-hop IP address instead of specifying Null0 as the interface.

Question 6

A network technician is troubleshooting a router-on-a-stick configuration. R1 has sub-interface G0/0.10 with encapsulation dot1q 10 and IP 192.168.10.1/24, and sub-interface G0/0.20 with encapsulation dot1q 20 and IP 192.168.20.1/24. Hosts in VLAN 10 cannot reach hosts in VLAN 20. The physical interface G0/0 is up and no shutdown. Both sub-interfaces show up/up. What should the technician do next?

Accepted Answer

Verify the switch port connected to R1 is configured as a trunk and allows VLANs 10 and 20.. The router's sub-interfaces being 'up/up' only confirms the physical link and Layer 2 protocol (using the native VLAN untagged, often VLAN 1) are active. It does not prove that the switch port is a trunk, nor that VLANs 10 and 20 are allowed across the link. Without a properly configured trunk, tagged frames for those VLANs will not traverse. Verifying the switch port trunk configuration and allowed VLANs is the most logical next step to resolve inter-VLAN communication at Layer 2 before investigating Layer 3 settings.

Answer

Verify the encapsulation dot1Q numbers on the sub-interfaces match the VLAN assignments.

Answer

Verify the default gateway settings on hosts in VLANs 10 and 20.

Answer

Check the physical interface G0/0 for interface errors or duplex mismatches.

Question 7

A technician configures OSPF on R1 using the command network 10.0.0.0 0.0.0.255 area 0. R1's GigabitEthernet0/0 interface has IP address 10.0.0.1/30 and is included in the OSPF process. The technician confirms the interface is not passive using the show ip ospf interface GigabitEthernet0/0 command. However, R2 is not forming an OSPF adjacency with R1. What should the technician do next?

Accepted Answer

Verify the subnet mask configured on R2's connecting interface.. On broadcast multi-access networks like Ethernet, OSPF requires both neighbors to agree on the exact subnet mask; a mismatch causes the hello packets to be rejected. R1's interface is already confirmed to be in the correct area and not passive, so the next logical step is to check R2's interface configuration. Verifying the subnet mask on R2's connecting interface addresses a fundamental Layer 3 misconfiguration that would directly prevent adjacency formation, before investigating OSPF-specific parameters.

Answer

Check the OSPF hello and dead intervals on R1 and R2.

Answer

Verify the OSPF area ID configured on R2's interface.

Answer

Check whether R2's interface is configured as a passive interface.

Question 8

Refer to the exhibit. An engineer expects all traffic destined to the 10.10.10.0 network to be forwarded via 10.1.1.2. However, when testing connectivity to host 10.10.10.64, traffic is being sent to 10.2.2.2 instead. What is the most likely cause?

Accepted Answer

The router is using longest prefix matching; the /26 route is more specific, so traffic to 10.10.10.64 is forwarded via 10.2.2.2.. The show ip route output contains two static routes for destination 10.10.10.0, one with a /24 prefix and one with a /26 prefix. Both routes have an administrative distance of 1 and a metric of 0, as indicated by the [1/0] in the output. The router always performs longest prefix matching; a packet destined to 10.10.10.64 falls within the range 10.10.10.0 – 10.10.10.63 of the /26 route, making the /26 the more specific match. Therefore, traffic to 10.10.10.64 is forwarded via 10.2.2.2 despite the engineer's intent.

Answer

The static route to 10.10.10.0/24 has a higher administrative distance than the /26 route, making the /26 preferred.

Answer

The /26 static route was configured as a floating static route with an administrative distance of 5, overriding the /24 route.

Answer

The next-hop 10.1.1.2 for the /24 route is unreachable, causing the router to fall back to the /26 route.

Question 9

R1 cannot reach host 10.3.3.1 on R3. The technician checks routing: R1 has a route to 10.3.3.0/24 via next-hop 10.1.1.2 (R2). R2 has a route to 10.3.3.0/24 via next-hop 10.2.2.2 (R3). A ping from R1 to 10.3.3.1 times out. A ping from R2 to 10.3.3.1 succeeds. What should the technician do next?

Accepted Answer

Check whether an inbound ACL on R3 is blocking packets with R1’s source IP address.. Because R2 can successfully ping R3, we know that R3 is reachable, the path from R2 to R3 is functional, OSPF (or whatever IGP) is working between them, and R3 has a route back to R2's subnet. The failure is isolated to traffic sourced from R1. This strongly suggests a filtering issue that specifically denies R1's source IP address. Checking for an inbound ACL on R3’s receiving interface is the logical next step at the transport/application layer; it directly tests the hypothesis that R3 is receiving R1’s pings but discarding them due to a security policy. It avoids unnecessary investigation of routing or link-layer problems that have already been ruled out.

Answer

Verify that R3 has a route back to R1’s subnet.

Answer

Verify the OSPF neighbor adjacency between R2 and R3.

Answer

Test for an MTU mismatch along the path from R1 to R3.

Question 10

Refer to the exhibit. A network engineer is verifying OSPF routing on R1. All routers in the topology are expected to reside in OSPF area 0, and the network 172.16.0.0/16 should be advertised from R2 within the same area. The engineer issues the show ip route 172.16.0.0 command on R1 and sees the output in the exhibit. The engineer expected to see an intra-area route (O) instead of an inter-area route (O IA). What is the most likely cause of this discrepancy?

Accepted Answer

The destination network 172.16.0.0/16 is located in a different OSPF area.. The exhibit explicitly lists the route type as 'inter area' for 172.16.0.0/16. In OSPF, an inter-area (IA) route means the destination network resides in a different OSPF area than the local router. Since all routers are expected to be in area 0, the presence of an IA route indicates the 172.16.0.0/16 network is actually located in a different area, causing the ABR to generate a Type-3 summary LSA.

Answer

R1 has a mismatched OSPF process ID compared to the ABR.

Answer

The network type on GigabitEthernet0/0 is configured as non-broadcast, preventing full adjacency.

Answer

The metric for the route is too high, causing OSPF to treat it as an inter-area route.

Question 11

A network engineer notices that an OSPF adjacency between R1 and R2 is flapping between FULL and DOWN state every 40 seconds. The dead interval on both routers is configured as 40 seconds. The hello interval on R1 is 10 seconds, and on R2 it is 30 seconds. What is the most likely cause?

Accepted Answer

Mismatched hello intervals cause the dead timer on R1 to expire before receiving a hello from R2, tearing down the adjacency.. The mismatched hello intervals cause the dead timer on R1 to expire before receiving a hello from R2. R1 sends hellos every 10 seconds and expects to hear from R2 at least once every 40 seconds (dead interval). R2, however, only sends hellos every 30 seconds. Due to timing jitter and processing delays, R1's dead timer occasionally expires just before R2's hello arrives, leading to the adjacency being torn down. After the adjacency drops, OSPF attempts to re-establish it, resulting in a cyclical FULL-DOWN pattern every 40 seconds. The other options are real OSPF issues but do not produce this specific timing: duplicate router IDs would cause persistent instability without a regular interval; an authentication mismatch would prevent the adjacency from forming at all; a network type mismatch might cause DR/BDR election problems but would not flap with precise dead-interval regularity.

Answer

Duplicate OSPF router IDs are causing the adjacency to reset each time the mismatched routers see each other's LSAs.

Answer

A network type mismatch between broadcast and point-to-point is causing periodic DR/BDR elections that reset the adjacency.

Answer

An OSPF authentication mismatch is causing periodic rejection of hello packets on one router.

Question 12

Refer to the exhibit. A network engineer is troubleshooting connectivity to server 10.10.10.130. The routing table contains both a static route and an OSPF route for overlapping prefixes. The engineer examines the specific routing entry for 10.10.10.130. Based on the output, why does the router choose the route via 10.1.1.2 instead of the OSPF route via 10.2.2.2 (for 10.10.10.0/24)?

Accepted Answer

The static route has a longer prefix length (/26) than the OSPF route (/24), making it a more specific match.. The exhibit shows "Routing entry for 10.10.10.128/26", a /26 prefix. The router uses longest prefix match as the first step in route selection, so a /26 is more specific than the OSPF /24 and chosen regardless of AD or metric.

Answer

The static route has a lower administrative distance (1) than the OSPF route (110).

Answer

The OSPF route is inactive because its next-hop 10.2.2.2 is down.

Answer

The router prefers the static route because it has a metric of 0, which is better than the OSPF metric.

Question 13

Refer to the exhibit. An administrator is troubleshooting connectivity from a branch router R1 to the internet. A ping to 8.8.8.8 from R1 fails. The output of the show ip route command is shown. What is the most likely cause?

Accepted Answer

The next-hop IP address for the default route is not reachable.. The routing table shows the static default route 'S* 0.0.0.0/0 [1/0] via 203.0.113.1', but there is no connected or static route to network 203.0.113.0/24 (or any subnet containing 203.0.113.1). Without a route to the next-hop, the default route is unreachable, so the router cannot forward traffic to the internet, causing ping failures.

Answer

The default route is missing from the routing table.

Answer

The static route has an incorrect administrative distance.

Answer

The ip routing process has not been enabled.

Question 14

Refer to the exhibit. A network administrator is troubleshooting why not all OSPF neighbors are fully adjacent on a multi-access broadcast segment. After issuing the show ip ospf neighbor command on R1, the output is displayed. What is the most likely cause of the 2WAY/DROTHER state for neighbor 172.16.1.1?

Accepted Answer

The neighbor is a DROther and a full adjacency with another DROther is not required; adjacency is formed only with the DR and BDR.. The output shows neighbor 172.16.1.1 in state 2WAY/DROTHER. On a broadcast multi-access network (indicated by the presence of DR and BDR roles), OSPF routers that are not the DR or BDR become DROthers. DROthers reach the 2-Way state with one another but do not form full adjacencies; full adjacency is established only with the DR and BDR. Therefore, the 2WAY state is normal and expected for two DROthers. The output explicitly displays the neighbor role as DROTHER, confirming this interpretation.

Answer

The OSPF hello and dead timers are mismatched between R1 and neighbor 172.16.1.1.

Answer

The network type is configured as point-to-point on one side and broadcast on the other, causing a DR/BDR election failure.

Answer

The neighbor's router ID 172.16.1.1 is not reachable, preventing the completion of the adjacency process.

Question 15

Refer to the exhibit. A network engineer is troubleshooting a missing route on R1. The router R3 is configured to advertise network 192.168.30.0/24 via OSPF, but the route is not present in the OSPF routing table of R1 when issuing the show ip route ospf command. What is the most likely cause?

Accepted Answer

R3 has passive-interface default configured and has not issued the no passive-interface command on the interface facing the OSPF neighbor.. The exhibit from R1# show ip route ospf displays OSPF routes for 192.168.10.0/24 and 192.168.20.0/24 learned via 10.1.1.2 on GigabitEthernet0/1, proving that OSPF is functioning with neighbor R2. The route 192.168.30.0/24 that R3 is supposed to advertise is missing, indicating that R1 has not formed an OSPF adjacency with R3. When 'passive-interface default' is configured, OSPF hello packets are suppressed on all interfaces unless explicitly overridden with 'no passive-interface'. As a result, R3 does not send hellos, preventing adjacency and route advertisement. This directly explains why the exhibit lacks the 192.168.30.0/24 entry.

Answer

R3 has not enabled OSPF on the network 192.168.30.0/24 using the network command.

Answer

R1 has an inbound ACL applied to GigabitEthernet0/1 that blocks OSPF multicast traffic to 224.0.0.5 and 224.0.0.6.

Answer

R3's OSPF router ID is duplicated with another OSPF router, causing the SPF algorithm to drop routes.