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 PracticeDuring a network migration from EIGRP to OSPF, you notice that some routes are being redistributed incorrectly, causing routing loops. The OSPF domain uses area 0 and area 1. The EIGRP domain uses AS 100. Which configuration change would best prevent loops during the migration?
Explanation: Route-maps allow you to tag redistributed EIGRP routes with a specific tag value (e.g., 'tag 100') and then filter those tagged routes on OSPF routers using a distribute-list in or prefix-list combined with the route-map. This prevents the redistributed routes from being re-injected back into EIGRP, breaking the redistribution loop. Without such tagging and filtering, mutual redistribution between EIGRP and OSPF can cause routing loops due to the two-way redistribution of routes.
Which OSPF network type elects a Designated Router (DR) and Backup Designated Router (BDR) to reduce the number of adjacencies on a multi-access segment?
Explanation: The OSPF broadcast network type is designed for multi-access segments like Ethernet, where multiple routers can communicate directly. On broadcast networks, OSPF elects a Designated Router (DR) and Backup Designated Router (BDR) to reduce the number of adjacencies from O(n²) to O(n), since all non-DR routers form adjacencies only with the DR and BDR. This is the defining behavior of the broadcast network type.
Drag and drop the steps to configure OSPF on a Cisco IOS router into the correct order.
Explanation: OSPF configuration requires entering global config mode, enabling the OSPF process, and then advertising networks under the OSPF process.
An SP network uses OSPF with RSVP-TE for traffic engineering. Which OSPF extension is required to advertise TE link parameters such as maximum reservable bandwidth?
Explanation: OSPF TE extensions are defined in RFC 3630, which introduces the Opaque LSA (Type 9, 10, 11). Specifically, Type 10 Opaque LSAs carry TE information. The TE LSA is carried in an Opaque LSA of area scope (Type 10).
A service provider is designing an OSPF network with multiple areas. Which TWO area types allow external routes (Type 5 LSAs) to be injected into the area? (Choose two.)
Explanation: Option B (Standard area) is correct because a standard OSPF area accepts all LSA types, including Type 5 external LSAs originated by ASBRs, so external routes can be injected and flooded throughout it. Option D (Backbone area, Area 0) is correct because the backbone is a normal (non-stub) area that permits Type 5 LSAs; in fact, external routes are typically injected at an ASBR and propagated across Area 0 to other areas. The unmarked options do not belong: an NSSA (A) blocks Type 5 LSAs and instead uses Type 7 LSAs that are translated to Type 5 only at the ABR, a stub area (C) blocks Type 5 LSAs entirely, and a totally stubby area (E) blocks Type 5 LSAs as well as Type 3 and Type 4 LSAs, relying solely on a default route.
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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 350-501 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 350-501. Practicing each scenario type ensures you're ready for any format.
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