300-410 OSPF Troubleshooting (v2/v3) Practice Questions

Explanation: The default OSPF network type on Ethernet interfaces is BROADCAST, which requires a DR/BDR election and uses multicast 224.0.0.5 and 224.0.0.6. On a point-to-point link, if both routers do not agree on the network type, they may not form an adjacency because the hello packets are sent differently. However, since both are BROADCAST, they should form an adjacency. The real issue is likely a mismatch in OSPF network type if one side is manually set to POINT-TO-POINT, but here both are BROADCAST. A more common cause is that the interface is administratively down or there is a Layer 1 issue, but the stem indicates the interface is up. Another common cause is that the router IDs are not configured, leading to OSPF not starting. The most likely cause in this scenario is that the routers have duplicate router IDs, which prevents adjacency formation. The stem does not mention router IDs, so the best answer is that the engineer should check for duplicate router IDs.

Explanation: In OSPFv3, the network type on serial interfaces defaults to POINT-TO-POINT, but if the interface is configured as BROADCAST (e.g., using 'ipv6 ospf network broadcast'), a DR/BDR election occurs. However, the adjacency is FULL, so the issue is not adjacency. The lack of OSPF routes in the routing table suggests that the routes are not being installed. One common reason is that OSPFv3 requires an IPv6 address on the interface, but the router may have an IPv4-only configuration or the interface may not have an IPv6 address. Another possibility is that the OSPFv3 process is not redistributing connected routes. The most likely cause is that the interface does not have an IPv6 address configured, which is required for OSPFv3 to advertise prefixes.

Explanation: By default, OSPF does not redistribute connected routes unless explicitly configured. The ASBR must use the 'redistribute connected' command under the OSPF process to advertise directly connected networks. The static routes are being redistributed because they are matched by the 'redistribute static' command, but the connected network is not part of the static route set unless it is also a static route. The engineer likely forgot to add 'redistribute connected' or use the 'subnets' keyword.

Explanation: On Frame Relay point-to-point subinterfaces, the OSPF network type should be point-to-point, but the adjacency may still fail if the Layer 2 mapping is incorrect. The subinterface must be mapped to the correct DLCI. If the 'frame-relay interface-dlci' command is missing or misconfigured, the router cannot send packets to the neighbor, even though OSPF is enabled. The engineer should verify the DLCI mapping.

Explanation: In OSPFv3, external routes are not installed if the router has a route to the forwarding address that is not reachable. However, the more common cause is that the OSPFv3 process on R1 has 'no ipv6 unicast-routing' or the router is missing a default route to reach the external destinations. Another possibility is that the external routes have a metric that is too high or the route is suppressed by a filter. The most likely cause is that the router has a route map or distribute list that is filtering the external routes from being installed.