OSPF Adjacency State Machine

Hi All,

The Hello Protocol is responsible for neighbour acquisition, maintenance and for ensuring two way communication between neighbours.

In other words, Hello packets are used to form neighbour/adjacency relationship on OSPF enabled links.

There are 8 STATE process to establish adjacency.

Down

Attempt

Init

2-Way

Exstart

Exchange

Loading

Full

DOWN

> Initial state of neighbour conversation.

> no Hellos received on OSPF enabled interface in the last RouterDeadInterval (40 Seconds by default – same as dead interval), but can send Hello to the neighbour.

ATTEMPT

> Only used for manually configured NBMA networks.

> A Router sends unicast Hello packet to OSPF enabled interface, but not received any.

INIT

> Hello packet has been seen by the neighbour, But my Router-ID has not been listed in neighbour list.

2-WAY

> This is the first step of adjacency establishment.

> Bi-directional Communication happen between two Routers.

> Hello packet has been seen by the neighbour in last RouterDeadInterval and my Router-ID is now listed in neighbour list.

> BDR and DR is elected in this state.

> If network type is Broadcast or Non-Broadcast Multi-Access (NBMA) then Router becomes FULL, only with DR and BDR, and it stays in 2-WAY state with remaining routers.

> If network type is Point-to-Point or Point-to-Multipoint then Router become FULL with connected Routers.

EXSTART

> Election happens to decide who will be the first, between pair of Routers, to share the database and whose sequence number will follow for future communication.

> Highest Router-ID wins the election and becomes the Master. This election process is on per neighbour basis.

> Sequence number of Master will follow and it acknowledged by Slave.

EXCHANGE

> Master Router starts exchanging DBD packets and it contains only LSA headers.

> Each DBD packet has sequence number and it is acknowledged by Slave Router.

> Link State Requests (LSR) can also be sent in this state for recent information.

LOADING

> Link State Updates (LSU) received from neighbour routers which was asked in EXCHANGE state.

FULL

> Neighbours are fully adjacent in this state. show ip osfp database command can be used to view all received LSAs.

#DV

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