This question is about the formation of OSPF adjacency. An OSPF adjacency will not form
correctly across a point-to-point link in the same area. Which would most likely cause this
problem?

A.
Each interface has a different OSPF cost.

B.
Each interface is configured with secondary addresses as well as primary addresses.

C.
Each interface has a different MTU size.

D.
Each interface is configured with the ip unnumbered loopback 0 command.

Explanation:
Unequal MTU means stuck in EX-START
The states are Down, Attempt, Init, 2-Way, Exstart, Exchange, Loading, and Full.
Down This is the first OSPF neighbor state. It means that no information (hellos) has been
received from this neighbor, but hello packets can still be sent to the neighbor in this state.
During the fully adjacent neighbor state, if a router doesn’t receive hello packet from a neighbor
within the RouterDeadInterval time (RouterDeadInterval = 4*HelloInterval by default) or if the
manually configured neighbor is being removed from the configuration, then the neighbor state
changes from Full to Down.
Attempt
This state is only valid for manually configured neighbors in an NBMA environment. In Attempt
state, the router sends unicast hello packets every poll interval to the neighbor, from which hellos
have not been received within the dead interval.
Init
This state specifies that the router has received a hello packet from its neighbor, but the receiving
router’s ID was not included in the hello packet. When a router receives a hello packet from a
neighbor, it should list the sender’s router ID in its hello packet as an acknowledgment that it
received a valid hello packet.
2-Way
This state designates that bi-directional communication has been established between two
routers. Bidirectional means that each router has seen the other’s hello packet. This state is
attained when the router receiving the hello packet sees its own Router ID within the received
hello packet’s neighbor field. At this state, a router decides whether to become adjacent with this
neighbor. On broadcast media and non-broadcast multiaccess networks, a router becomes full
only with the designated router (DR) and the backup designated router (BDR); it stays in the 2-way
state with all other neighbors. On Point-to-point and Point-to-multipoint networks, a router
becomes full with all connected routers.
At the end of this stage, the DR and BDR for broadcast and non-broadcast multiacess networks
are elected.
For more information on the DR election process, refer to DR Election.
Note: Receiving a Database Descriptor (DBD) packet from a neighbor in the init state will also a
cause a transition to 2-way state.
Exstart

Once the DR and BDR are elected, the actual process of exchanging link state information can
start between the routers and their DR and BDR.
In this state, the routers and their DR and BDR establish a master-slave relationship and choose
the initial sequence number for adjacency formation. The router with the higher router ID becomes
the master and starts the exchange, and as such, is the only router that can increment the
sequence number. Note that one would logically conclude that the DR/BDR with the highest router
ID will become the master during this process of master-slave relation. Remember that the
DR/BDR election might be purely by virtue of a higher priority configured on the router instead of
highest router ID. Thus, it is possible that a DR plays the role of slave. And also note that
master/slave election is on a per-neighbor basis.
Exchange
In the exchange state, OSPF routers exchange database descriptor (DBD) packets. Database
descriptors contain link-state advertisement (LSA) headers only and describe the contents of the
entire link-state database. Each DBD packet has a sequence number which can be incremented
only by master which is explicitly acknowledged by slave. Routers also send link-state request
packets and link-state update packets (which contain the entire LSA) in this state. The contents of
the DBD received are compared to the information contained in the routers link-state database to
check if new or more current link-state information is available with the neighbor.
Loading
In this state, the actual exchange of link state information occurs. Based on the information
provided by the DBDs, routers send link-state request packets. The neighbor then provides the
requested link-state information in link-state update packets. During the adjacency, if a router
receives an outdated or missing LSA, it requests that LSA by sending a link-state request packet.
All link-state update packets are acknowledged.
Full
In this state, routers are fully adjacent with each other. All the router and network LSAs are
exchanged and the routers’ databases are fully synchronized.
Full is the normal state for an OSPF router. If a router is stuck in another state, it’s an indication
that there are problems in forming adjacencies. The only exception to this is the 2-way state,
which is normal in a broadcast network. Routers achieve the full state with their DR and BDR only.
Neighbors always see each other as 2-way.
Troubleshooting OSPF Neighbor Relationships
OSPF Neighbor List is Empty
OSPF not enabled properly on appropriate interfaces
Layer 1 or 2 not functional
Passive interface configured
Access list(s) blocking Hello packets in multiple directions

Error in IP address or subnet mask configuration
Hello or dead interval mismatch
Authentication configuration error
Area ID mismatch
Stub flag mismatch
OSPF adjacency exists with secondary IP addressing or asynchronous interface
Incorrect configuration type for nonbroadcast multiaccess (NBMA) environment
OSPF Neighbor Stuck in Attempt
Misconfigured neighbor statement
Unicast nonfunctional in NBMA environment
OSPF Neighbor Stuck in init
Access list or Layer 2 problem blocking Hellos in one direction
Multicast nonfunctional on one side
Authentication configured on only one side
Broadcast keyword missing from the map command
OSPF Neighbor Stuck in Two-Way
Priority 0 configured on all routers
OSPF Neighbor Stuck in Exstart/Exchange
Mismatch interface maximum transmission unit (MTU)
Duplicate router IDs on routers
Broken unicast connectivity
Network type of point-to-point between Primary Rate Interface (PRI) and Basic Rate Interface
(BRI)/dialer
OSPF Neighbor Stuck in Loading
Mismatched MTU
Corrupted link-state request packet

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Cisco General Networking Theory Quick Reference Sheets