Refer to the graphic.
R1 is unable to establish an OSPF neighbor relationship with R3. What are possible reasons for
this problem? (Choose two.)
A.
All of the routers need to be configured for backbone Area 1.
B.
R1 and R2 are the DR and BDR, so OSPF will not establish neighbor adjacency with R3.
C.
A static route has been configured from R1 to R3 and prevents the neighbor adjacency from
being established.
D.
The hello and dead interval timers are not set to the same values on R1 and R3.
E.
EIGRP is also configured on these routers with a lower administrative distance.
F.
R1 and R3 are configured in different areas.
A is not correct because the backbone area of OSPF is always Area 0.
B is not correct because R1 or R3 must be the DR or BDR -> it has to establish neighbor adjacency with the other.
C is not correct because OSPF neighbor relationship is not established based on static routing. It uses multicast address 224.0.0.5 to establish OSPF neighbor relationship.
E is not correct because configure EIGRP on these routers (with a lower administrative distance) will force these routers to run EIGRP, not OSPF.
D and F are correct because these entries must match on neighboring routers:
– Hello and dead intervals
– Area ID (Area 0 in this case)
– Authentication password
– Stub area flag
But the diagram indicates that it IS in areas 0….
The question asks for possible causes. The three routers are all in Area 0, so the possible problem is that either R1 or R3 is not configured for the correct area (Area 0).
OSPF uses the hello and dead interval to check if a remote neighbor is still alive.
OSPF routing devices constantly track the status of their neighbors, sending and receiving hello packets that indicate whether each neighbor still is functioning, and sending and receiving link-state advertisement (LSA) and acknowledgment packets. OSPF sends packets and expects to receive packets at specified intervals.
Hello interval — Routing devices send hello packets at a fixed interval on all interfaces, including virtual links, to establish and maintain neighbor relationships. The hello interval specifies the length of time, in seconds, before the routing device sends a hello packet out of an interface. This interval must be the same on all routing devices on a shared network. By default, the routing device sends hello packets every 10 seconds (broadcast and point-to-point networks) and 30 seconds (nonbroadcast multiple access (NBMA) networks).
Poll interval—(OSPFv2, Nonbroadcast networks only) Routing devices send hello packets for a longer interval on nonbroadcast networks to minimize the bandwidth required on slow WAN links. The poll interval specifies the length of time, in seconds, before the routing device sends hello packets out of the interface before establishing adjacency with a neighbor. By default, the routing device sends hello packets every 120 seconds until active neighbors are detected.
Once the routing device detects an active neighbor, the hello packet interval changes from the time specified in the poll interval to the time specified in the hello interval.
LSA retransmission interval—When a routing device sends LSAs to its neighbors, the routing device expects to receive an acknowledgment packet from each neighbor within a certain amount of time. The LSA retransmission interval specifies the length of time, in seconds, that the routing device waits to receive an LSA packet before retransmitting the LSA to an interface’s neighbors. By default, the routing device waits 5 seconds for an acknowledgment before retransmitting the LSA.
Dead interval—If a routing device does not receive a hello packet from a neighbor within a fixed amount of time, the routing device modifies its topology database to indicate that the neighbor is nonoperational. The dead interval specifies the length of time, in seconds, that the routing device waits before declaring that a neighboring routing device is unavailable. This is an interval during which the routing device receives no hello packets from the neighbor. This interval must be the same on all routing devices on a shared network. By default, this interval is four times the default hello interval, which is 40 seconds (broadcast and point-to-point networks) and 120 seconds (NBMA networks).
OSPF uses a DR (Designated Router) and BDR (Backup Designated Router) on each multi-access network.