Wednesday, 7 November 2018

OSPF Neighbor states


OSPF neighbor states



During the OSPF adjacency formation process, two OSPF routers transition through several states, which include:







  1. Down-  first OSPF neighbor state in this state no information (hello) has been received from the neighbouring router but in the down state hello packets can be sent to the neighbor.
  2. Attempt- this state is only valid for manually configured neighbor in an non broadcast multi-access (NBMA) environment. router send unicast hello packets every interval to the neighbor orm which hello's have not been received within the dead interval.
  3. Init-OSPF router recieved a hello packets from its neighbor but the receiving router's ID was not include in the hello packets.
Router ID (RID)- This is the highest active IP address on the router. Router ID is configured manually and dynamically if its not configured by the administrator, router choose the highest IP of the loopback interface is used and if loopback is not configured, Router choose the highest IP of physical interface.   

 Configure Router ID-

R1(config)#router ospf 1
R1(config-router)#router-id ?
  A.B.C.D  OSPF router-id in IP address format


4. 2 WAY- 2 way state is attained when the router receiving the hello packets and router seen its own router ID within the received  hello packet. Bidirectional communication has been established. In the broadcast multi-access networks, an election can occur after this point.




5. Exstart- the actual process of exchanging link states information can start between the OSPF  routers. master/slave relationship is established and initial sequence numbers are exchange.






6.Exchange- In the Exchange state, OSPF routing information is exchanged using Database description (DBD or DD) packets and link-state request (LSR) and link state update (LSU) packets may also be sent. In this state OSPF router see what LSA's they have and what LSA's they do not have.

7.Full- In the full state, all LSA's information is synchronized among neighbors and adjacency has been established. 

%OSPF-5-ADJCHG: Process 1, Nbr 20.1.1.1 on Serial3/0 from LOADING to FULL, Loading Done


R1#show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
20.1.1.1          0   FULL/  -        00:00:39    1.1.1.2         Serial3/0



Before we start  BGP configuration check out some important  of BGP CCIE exam topics.


Here are the lists of all OSPF labs and theories CCNA to CCIE.






























































































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