OSPF – The Interface State Machine

The Interface State Machine
An OSPF-enabled interface will transition through several states before it becomes fully functional.
Those states are Down, Point-to-Point, Waiting, DR, Backup, DRother, and Loopback.
Down.
This is the initial interface state. The interface is not functional, all interface parameters are set to their
initial values, and no protocol traffic is transmitted or received on the interface.
Point-to-Point.
This state is applicable only to interfaces connected to point-to-point, point-to-multipoint, and virtual link
network types. When an interface transitions to this state, it is fully functional. It will begin sending Hello
packets every HelloInterval and will attempt to establish an adjacency with the neighbor at the other end
of the link.
Waiting.
This state is applicable only to interfaces connected to broadcast and NBMA network types. When an
interface transitions to this state, it will begin sending and receiving Hello packets and will set the wait
timer. The router will attempt to identify the network’s DR and BDR while in this state.
DR.
In this state, the router is the DR on the attached network and will establish adjacencies with the other
routers on the multi-access network.
Backup.
In this state, the router is the BDR on the attached network and will establish adjacencies with the other
routers on the multi-access network.
DRother.
In this state, the router is neither the DR nor the BDR on the attached network. It will form adjacencies
only with the DR and BDR, although it will track all neighbors on the network.
Loopback.
In this state, the interface is looped back via software or hardware. Although packets cannot transit an
interface in this state, the interface address is still advertised in router LSAs (described later) so that test
packets can find their way to the interface.

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