When OSPF adjacency is formed, a router goes through several state
changes before it becomes fully adjacent with its neighbor. Those states are
defined in the OSPF
section 10.1. The states are Down, Attempt, Init, 2-Way, Exstart, Exchange,
Loading, and Full. This document describes each state in detail.
There are no specific requirements for this document.
This document is not restricted to specific software and hardware
For more information on document conventions, refer to
Technical Tips Conventions.
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.
This state is only valid for manually configured neighbors in an
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.
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.
This state designates that bi-directional communication has been
established between two routers. Bi-directional 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
Note: Receiving a Database Descriptor (DBD) packet from a neighbor in the
init state will also a cause a transition to 2-way state.
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.
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.
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.
In this state, routers are fully adjacent with each other. All the
router and network LSAs are exchanged and the routers' databases are fully
Full is the normal state for an OSPF router. If a router is stuck in
another state, it is 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 in
NBMA/broadcast media and FULL state with every neighbor in the remaining media
such as point-to-point and point-to-multipoint.
Note: The DR and BDR that achieve FULL state with every router on the
segment will be displayed as FULL/DROTHER when you enter the show ip
ospf neighbor command on either a DR or BDR. This simply means that
the neighbor is not a DR or BDR, but since the router on which the command was
entered is either a DR or BDR, this shows the neighbor as FULL/DROTHER.