OSPF – The Neighbor State Machine

Down.
The initial state of a neighbor conversation indicates that no Hellos have been heard from the neighbor in
the last RouterDeadInterval. Hellos are not sent to down neighbors unless those neighbors are on NBMA
networks; in this case, Hellos are sent every PollInterval. If a neighbor transitions to the Down state from
some higher state, the link state Retransmission List, Database Summary List, and link state request list
are cleared.
Attempt.
This state applies only to neighbors on NBMA networks, where neighbors are manually configured. A
DR-eligible router will transition a neighbor to the Attempt state when the interface to the neighbor first
becomes Active or when the router is the DR or BDR. A router will send packets to a neighbor in Attempt
state at the HelloInterval instead of the PollInterval.

Init.
This state indicates that a Hello packet has been seen from the neighbor in the last RouterDeadInterval,
but 2-Way communication has not yet been established. A router will include the Router IDs of all
neighbors in this state or higher in the Neighbor field of the Hello packets.

2-Way.
This state indicates that the router has seen its own Router ID in the Neighbor field of the neighbor’s
Hello packets, which means that a bidirectional conversation has been established. On multi-access
networks, neighbors must be in this state or higher to be eligible to be elected as the DR or BDR. The
reception of a Database Description packet from a neighbor in the init state will also cause a transition to
2-Way.

ExStart.
In this state, the router and its neighbor establish a master/slave relationship and determine the initial DD
sequence number in preparation for the exchange of Database Description packets. The neighbor with the
highest interface address becomes the master.

Exchange.
The router sends Database Description packets describing its entire link state database to neighbors that
are in the Exchange state. The router may also send Link State Request packets, requesting more recent
LSAs, to neighbors in this state.

Loading.
The router will send Link State Request packets to neighbors that are in the Loading state, requesting
more recent LSAs that have been discovered in the Exchange state but have not yet been received.

Full.
Neighbors in this state are fully adjacent, and the adjacencies will appear in Router LSAs and Network
LSAs.

Three flags in the DD packet are used to manage the adjacency building process:
1. The I-bit, or Initial bit, which when set indicates the first DD packet sent
2. The M-bit, or More bit, which when set indicates that this is not the last DD packet to be sent
3. The MS-bit, or Master/Slave bit, which is set in the DD packets originated by the master

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