Scenario
Refer to the topology. Your company has decided to connect the main office with three other
remote branch offices using point-to-point serial links.
You are required to troubleshoot and resolve OSPF neighbor adjacency issues between the main
office and the routers located in the remote branch offices.
R1 does not form an OSPF neighbor adjacency with R2. Which option would fix the issue?
A.
R1 ethernetO/1 is shutdown. Configure no shutdown command.
B.
R1 ethernetO/1 configured with a non-default OSPF hello interval of 25: configure no ip ospf
hello-interval 25
C.
R2 ethernetO/1 and R3 ethernetO/O are configured with a non-default OSPF hello interval of
25; configure no ip ospf hello-interval 25
D.
Enable OSPF for R1 ethernetO/1; configure ip ospf 1 area 0 command under ethernetO/1
OSPF hello packets are packets that an OSPF process sends to its OSPF neighbors to maintain connectivity with those neighbors. The hello packets are sent at a configurable interval (in seconds). The defaults are 10 seconds for an Ethernet link and 30 seconds for a non broadcast link. Hello packets include a list of all neighbors for which a hello packet has been received within the dead interval. The dead interval is also a configurable interval (in seconds), and defaults to four times the value of the hello interval. The value of all hello intervals must be the same within a network. Likewise, the value of all dead intervals must be the same within a network.
These two intervals work together to maintain connectivity by indicating that the link is operational. If a router does not receive a hello packet from a neighbor within the dead interval, it will declare that neighbor to be down.
OSPF Fast Hello Packets
OSPF fast hello packets refer to hello packets being sent at intervals of less than 1 second. To understand fast hello packets, you should already understand the relationship between OSPF hello packets and the dead interval. See the section “OSPF Hello Interval and Dead Interval” section.
OSPF fast hello packets are achieved by using the ip ospf dead-interval command. The dead interval is set to 1 second, and the hello-multiplier value is set to the number of hello packets you want sent during that 1 second, thus providing subsecond or “fast” hello packets.
When fast hello packets are configured on the interface, the hello interval advertised in the hello packets that are sent out this interface is set to 0. The hello interval in the hello packets received over this interface is ignored.
The dead interval must be consistent on a segment, whether it is set to 1 second (for fast hello packets) or set to any other value. The hello multiplier need not be the same for the entire segment as long as at least one hello packet is sent within the dead interval.