Ospf States

8/2/2019 Ospf States

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PANKAJ H RATHI


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OSPF NEIGHBOR STATES Down:

This is the first OSPF neighbor state. It means that noinformation (hellos) has been received from thisneighbor, but hello packets can still be sent to theneighbor in this state. During the fully adjacentneighbor state, if a router doesn't receive hello packetfrom a neighbor within the RouterDeadInterval time

(RouterDeadInterval = 4*Hello Interval by default) orif the manually configured neighbor is being removedfrom the configuration, then the neighbor statechanges from Full to Down.


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OSPF NEIGHBOR STATES CONT.Attempt:

This state is only valid for manually configured

neighbors in an NBMA environment. In Attempt state,the router sends unicast hello packets every pollinterval to the neighbor, from which hellos have notbeen received within the dead interval.


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OSPF NEIGHBOR STATES CONT. Init

This state specifies that the router has received a hello

packet from its neighbor, but the receiving router's IDwas not included in the hello packet. When a routerreceives a hello packet from a neighbor, it should listthe sender's router ID in its hello packet as an

acknowledgment that it received a valid hello packet


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OSPF NEIGHBOR STATES CONT.2-WAY This state designates that bidirectional communication has been

established between two routers. Bidirectional means that each routerhas seen the other's hello packet. This state is attained when the router

receiving the hello packet sees its own Router ID within the receivedhello packet's neighbor field. At this state, a router decides whether tobecome adjacent with this neighbor. On broadcast media andnonbroadcast multiaccess networks, a router becomes full only withthe designated router (DR) and the backup designated router (BDR); itstays in the 2way state with all other neighbors. On Pointtopointand Pointtomultipoint networks, a router becomes full with all

connected routers. At the end of this stage, the DR and BDR for broadcast and

nonbroadcast multiacess networks are elected. Note: Receiving a Database Descriptor (DBD) packet from a

neighbor in the init state will also a cause a transition to 2way state.


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OSPF NEIGHBOR STATES CONT. Exstart Once the DR and BDR are elected, the actual process of exchanging

link state information can start between the routers and their DR andBDR.

In this state, the routers and their DR and BDR establish a masterslaverelationship and choose the initial sequence number for adjacencyformation. The router with the higher router ID becomes the masterand starts the exchange, and as such, is the only router that canincrement the sequence number. Note that one would logicallyconclude that the DR/BDR with the highest router ID will become themaster during this process of masterslave relation. Remember thatthe DR/BDR election might be purely by virtue of a higher priorityconfigured on the router instead of highest router ID. Thus, it ispossible that a DR plays the role of slave. And also note thatmaster/slave election is on a perneighbor basis.


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OSPF NEIGHBOR STATES CONT- Exchange In the exchange state, OSPF routers exchange database

descriptor (DBD) packets. Database descriptors contain

linkstate advertisement (LSA) headers only and describethe contents of the entire linkstate database. Each DBDpacket has a sequence number which can be incrementedonly by master which is explicitly acknowledged by slave.Routers also send linkstate request packets and linkstateupdate packets (which contain the entire LSA) in this state.

The contents of the DBD received are compared to theinformation contained in the routers linkstate database tocheck if new or more current linkstate information isavailable with the neighbor


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OSPF NEIGHBOR STATES CONT- Loading

In this state, the actual exchange of link state

information occurs. Based on the informationprovided by the DBDs, routers send linkstate requestpackets. The neighbor then provides the requestedlinkstate information in linkstate update packets.

During the adjacency, if a router receives an outdatedor missing LSA, it requests that LSA by sending alinkstate request packet. All linkstate updatepackets are acknowledged


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OSPF NEIGHBOR STATES CONT- Full

In this state, routers are fully adjacent with each other.All the router and network LSAs are exchanged and

the routers' databases are fully synchronized. Full isthe normal state for an OSPF router. If a router is stuckin another state, it's an indication that there areproblems in forming adjacencies. The only exception

to this is the 2way state, which is normal in abroadcast network. Routers achieve the full state withtheir DR and BDR only. Neighbors always see eachother as 2way.

Ospf States

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