CCNA IP Routing Practice Question

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

• ✓

  OSPF uses cost as its metric, which is derived from the bandwidth of the interface.

  Why this is correct

  OSPF uses a cost metric calculated from the interface bandwidth, specifically cost = reference-bandwidth / interface-bandwidth, with the default reference bandwidth on Cisco routers being 100 Mbps. Because cost is inversely proportional to bandwidth, a faster interface yields a lower cost, making it more preferred. The cumulative cost along a path determines the best route, and mismatched reference bandwidths across routers can cause suboptimal routing.

• ✓

  Hello packets are used to discover neighbors and maintain adjacencies.

  Why this is correct

  Hello packets in OSPF are multicast (224.0.0.5) every 10 seconds on broadcast networks and 30 seconds on non-broadcast networks to discover neighbors and maintain adjacencies. They carry crucial parameters such as Router ID, hello/dead intervals, area ID, and authentication data, and are used to elect the DR/BDR. Receiving a Hello from a peer establishes a two-way state, after which database description packets proceed before an adjacency fully forms.

• ✓

  The designated router (DR) is elected on broadcast multiaccess networks to reduce LSAs flooding.

  Why this is correct

  On broadcast multiaccess segments like Ethernet, OSPF elects a Designated Router (DR) and Backup Designated Router (BDR) to reduce LSA flooding and adjacency count. All routers form full adjacencies only with the DR and BDR, while maintaining two-way state among themselves, reducing the number of full adjacencies from O(n^2) to O(2n). The DR then originates a Type 2 Network LSA to summarize the segment, and BDR takes over if the DR fails.

• ✗

  OSPFv2 supports IPv6 routing natively without any additional configuration.

  Why it's wrong here

  OSPFv2 is specifically designed for IPv4 and cannot route IPv6 natively; routing IPv6 requires OSPFv3, which runs over IPv6 link-local addresses and has a different packet format and LSA types. OSPFv3 also supports multiple address families per instance, which OSPFv2 lacks. Thus, the statement claiming OSPFv2 supports IPv6 without additional configuration is false.

• ✗

  Link-state advertisements (LSAs) are sent periodically every 30 seconds by default.

  Why it's wrong here

  OSPF does not send LSAs periodically every 30 seconds; the 30-second interval is the RIP routing update timer. Instead, OSPF sends LSAs only when a topology change occurs, and every 30 minutes an LSA is refreshed to maintain sequence numbers and prevent refresh of the entire database too frequently. Periodic flooding every 30 seconds would waste bandwidth and cause unnecessary SPF recalculations, which is exactly what OSPF avoids.

• ✗

  OSPF routers in the same area must be configured with the same router ID.

  Why it's wrong here

  Router IDs in OSPF must be unique across the entire OSPF domain, not identical, for proper route identification and to prevent neighbor adjacency problems. Routers in the same area can and typically do have different router IDs, either manually configured or derived from the highest loopback address or active interface IP. Requiring the same router ID would be nonsensical and cause duplicate-RID conflicts, which are treated as routing errors.

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