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 PracticeWhat is the default administrative distance for OSPF routes on a Cisco IOS-XE router?
Explanation: The default administrative distance for OSPF is 110.
A network engineer runs the following command to troubleshoot route redistribution: R1# debug ip routing IP: route table insert (10.10.10.0/24 via 192.168.1.1, ospf 1) metric [110/20] IP: route table insert (10.10.10.0/24 via 10.1.1.2, eigrp 100) metric [90/158720] IP: route table delete (10.10.10.0/24 via 192.168.1.1, ospf 1) metric [110/20] IP: route table insert (10.10.10.0/24 via 10.1.1.2, eigrp 100) metric [90/158720] What does this output indicate?
Explanation: The debug output shows the OSPF route (AD 110) being inserted first, then the EIGRP route (AD 90) being inserted, followed by the deletion of the OSPF route and re-insertion of the EIGRP route. This sequence indicates that the EIGRP route replaces the OSPF route because EIGRP has a lower administrative distance (90 vs. 110), making it more trustworthy. The routing table only keeps the best route based on AD, not metric, when comparing routes from different protocols.
A network engineer runs the following command to troubleshoot an OSPF adjacency issue: R1# debug ip ospf adj OSPF: Interface GigabitEthernet0/0 going Up OSPF: Send with youngest orig age 0 OSPF: Rcv DBD from 2.2.2.2 seq 0x1A opt 0x52 flag 0x7 len 32 mtu 1500 state INIT OSPF: First DBD and we are not SLAVE OSPF: Rcv DBD from 2.2.2.2 seq 0x1A opt 0x52 flag 0x7 len 32 mtu 1500 state EXSTART OSPF: Nbr 2.2.2.2 has larger interface MTU What does this output indicate?
Explanation: The debug output shows that R1 receives a Database Description (DBD) packet from neighbor 2.2.2.2 with the INIT and EXSTART states, and then explicitly states 'Nbr 2.2.2.2 has larger interface MTU'. This indicates an MTU mismatch where the neighbor's interface MTU (1500) is larger than R1's interface MTU, preventing the OSPF adjacency from progressing beyond the EXSTART state. Option B is correct because the adjacency is stuck due to the MTU mismatch, as OSPF requires matching MTU values on both sides for DBD exchange to complete.
Given the following configuration on a router: ``` router ospf 1 distance 150 ``` What is the effect of this configuration?
Explanation: This command sets the administrative distance for all OSPF routes to 150, overriding the default of 110.
Given this configuration on router R2: ``` ip vrf CUSTOMER_D rd 100:1 ! interface GigabitEthernet0/0 ip vrf forwarding CUSTOMER_D ip address 192.168.2.1 255.255.255.0 ! router ospf 1 vrf CUSTOMER_D network 192.168.2.0 0.0.0.255 area 0 ``` What will happen when this configuration is applied?
Explanation: The OSPF process is tied to VRF CUSTOMER_D, so it only runs on interfaces that belong to that VRF. The network command matches the interface, so OSPF will form adjacencies on that interface within the VRF.
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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 300-410 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 300-410. Practicing each scenario type ensures you're ready for any format.
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