OSPF neighbour adjacencies, route advertisements, and DR/BDR elections appear consistently on the CCNA. These questions test whether you can read OSPF state from show commands and identify why two routers fail to reach FULL adjacency or why a route isn't being learned.
Start Scenario PracticeDrag and drop the steps of OSPF summarization at ABR configuration steps into the correct order, from first to last.
Explanation: First, identify the subnets to summarize into a single prefix. Then, configure the area range command on the ABR under the OSPF process, specifying the area and the summary prefix. Optionally, set the 'not-advertise' keyword to suppress the summary. Verify the summary route in the OSPF database using 'show ip ospf summary-address'. Finally, check that the summary route appears in the routing table of other routers.
What is the default OSPF hello interval on an Ethernet link in a Cisco router?
Explanation: The default OSPF hello interval on an Ethernet link (which is a broadcast multi-access network type) is 10 seconds. This is defined in RFC 2328 and is the default on Cisco routers for broadcast and point-to-point network types. The hello interval determines how often a router sends hello packets to discover and maintain neighbor relationships.
An engineer is troubleshooting an MPLS VPN where CE1 (10.1.1.0/24) cannot reach CE2 (10.2.2.0/24). The PE routers are running OSPF with the CE routers. On PE1, the 'show ip route vrf CUSTOMER' output shows 10.2.2.0/24 as an OSPF route, but the prefix is not present in the global BGP table. What is the most likely cause?
Explanation: In an MPLS VPN, the PE router must redistribute OSPF routes learned from the CE into MP-BGP under the VRF to propagate them across the MPLS backbone. Without this redistribution, the prefix 10.2.2.0/24 appears in the VRF routing table as an OSPF route but is never injected into the BGP table, so it cannot be advertised to the remote PE. This explains why CE1 cannot reach CE2 despite the route being present locally on PE1.
A network engineer runs the following command on Router R1: R1# show ip route vrf CUSTOMER-A VRF CUSTOMER-A: Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2 i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2 ia - IS-IS inter area, * - candidate default, U - per-user static route o - ODR, P - periodic downloaded static route Gateway of last resort is 10.0.1.1 to network 0.0.0.0 10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks C 10.0.0.0/30 is directly connected, GigabitEthernet0/0.100 L 10.0.0.1/32 is directly connected, GigabitEthernet0/0.100 B 10.0.2.0/24 [200/0] via 192.168.1.2, 00:12:34 Based on this output, what can be concluded?
Explanation: The output shows a BGP-learned route to 10.0.2.0/24 with the code 'B' and the administrative distance [200/0], indicating it is an external BGP route. The route is installed in the VRF CUSTOMER-A routing table, confirming that VRF CUSTOMER-A is using BGP and has learned this prefix via BGP from the next-hop 192.168.1.2.
Examine the following configuration snippet: interface GigabitEthernet0/1 ip address 192.168.1.1 255.255.255.0 ip ospf network point-to-point ip ospf hello-interval 10 ip ospf dead-interval 40 ! router ospf 1 network 192.168.1.0 0.0.0.255 area 0 What is the effect of this configuration?
Explanation: The configuration sets the OSPF network type to point-to-point on the interface, which allows OSPF to form an adjacency with a neighbor using the configured hello interval of 10 seconds and dead interval of 40 seconds. These intervals are valid for point-to-point networks, and the network command under router ospf correctly enables OSPF on the interface. Therefore, an adjacency will form as long as the neighbor's intervals match.
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Practice all OSPF Troubleshooting ScenariosOSPF neighbour adjacencies, route advertisements, and DR/BDR elections appear consistently on the CCNA. These questions test whether you can read OSPF state from show commands and identify why two routers fail to reach FULL adjacency or why a route isn't being learned. These appear throughout the 350-401 and require you to apply your knowledge, not just recall facts.
Cisco doesn't publish an exact breakdown, but scenario-based questions (especially exhibit and command-output formats) make up a significant portion of the 350-401. Practicing each scenario type ensures you're ready for any format.
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