OSPF State
The adjacency building process takes effect after multiple stages have been fulfilled. Routers that
become adjacent will have the exact link-state database. The following is a brief summary of the
states an interface passes through before becoming adjacent to another router:
Down: No information has been received from anybody on the segment.
Attempt: On non-broadcast multi-access clouds such as Frame Relay and X.25, this state
indicates that no recent information has been received from the neighbor. An effort should be
made to contact the neighbor by sending Hello packets at the reduced rate PollInterval.
Init: The interface has detected a Hello packet coming from a neighbor but bi-directional
communication has not yet been established.
Two-way: There is bi-directional communication with a neighbor. The router has seen itself in
the Hello packets coming from a neighbor. At the end of this stage the DR and BDR election
would have been done. At the end of the 2way stage, routers will decide whether to proceed
in building an adjacency or not. The decision is based on whether one of the routers is a DR
or BDR or the link is a point-to-point or a virtual link.
Exstart: Routers are trying to establish the initial sequence number that is going to be used in
the information exchange packets. The sequence number insures that routers always get the
most recent information. One router will become the primary and the other will become
secondary. The primary router will poll the secondary for information.
Exchange: Routers will describe their entire link-state database by sending database
description packets. At this state, packets could be flooded to other interfaces on the router.
Loading: At this state, routers are finalizing the information exchange. Routers have built a
link-state request list and a link-state retransmission list. Any information that looks incomplete
or outdated will be put on the request list. Any update that is sent will be put on the
retransmission list until it gets acknowledged.
Full: At this state, the adjacency is complete. The neighboring routers are fully adjacent.
Adjacent routers will have a similar link-state database.
The adjacency building process takes effect after multiple stages have been fulfilled. Routers that
become adjacent will have the exact link-state database. The following is a brief summary of the
states an interface passes through before becoming adjacent to another router:
Down: No information has been received from anybody on the segment.
Attempt: On non-broadcast multi-access clouds such as Frame Relay and X.25, this state
indicates that no recent information has been received from the neighbor. An effort should be
made to contact the neighbor by sending Hello packets at the reduced rate PollInterval.
Init: The interface has detected a Hello packet coming from a neighbor but bi-directional
communication has not yet been established.
Two-way: There is bi-directional communication with a neighbor. The router has seen itself in
the Hello packets coming from a neighbor. At the end of this stage the DR and BDR election
would have been done. At the end of the 2way stage, routers will decide whether to proceed
in building an adjacency or not. The decision is based on whether one of the routers is a DR
or BDR or the link is a point-to-point or a virtual link.
Exstart: Routers are trying to establish the initial sequence number that is going to be used in
the information exchange packets. The sequence number insures that routers always get the
most recent information. One router will become the primary and the other will become
secondary. The primary router will poll the secondary for information.
Exchange: Routers will describe their entire link-state database by sending database
description packets. At this state, packets could be flooded to other interfaces on the router.
Loading: At this state, routers are finalizing the information exchange. Routers have built a
link-state request list and a link-state retransmission list. Any information that looks incomplete
or outdated will be put on the request list. Any update that is sent will be put on the
retransmission list until it gets acknowledged.
Full: At this state, the adjacency is complete. The neighboring routers are fully adjacent.
Adjacent routers will have a similar link-state database.
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