OSPF neighbor states is the states that a router will go through in order to form a full relationship.
full relationship is called full adjacency, so it might happened that two routers have formed a neighbor relationship but because of some error they can’t proceed to full relationship. so they are neighbors but not full adjacent.
some routers are meant to stop at the 2-way state with some routers and full with other routers .
1: Down state
- if a router has a full relation with another, but for some reason they communication has stopped, the ospf will move the neighbor from full to down , generate a console message and remove it from the neighbors list
- this state will only be used for manually configured neighbors.
- it means that a router is trying to communicate with its neighbor by sending hello’s to it .
- hello’s that sent to manually configured neighbor are unicast , while normal hellos are multicast
- this state mean that a router received a hello from another router but his router id is not in the neighbor list inside that hello.
- as a result for this, the receiving router will list the sender router id on the neighbor field inside its hellos.
- A router will move to this state when the received hello include his router id on the neighbor field inside the hello
- Now both routers see each other hello’s, and they are ready to move to the next step.
The first step will be Dr/BDR election and these are the winning rules ( per segment ).
- highest priority ( default is 1 ) ( zero priority = don’t participate on the election )
- highest router id ( the sub point indicate how the router id get assigned )
- hardcode via command.
- highest loop back address.
- highest physical addres.
- unfortunately these election rules will only happen in the la la land, or by a magic way you managed to start all devices at the same time.
- because first router will come online will elect itself as DR, and election will not happen again for that segment , unless of course the DR/BDR failed . and that’s why you use tweaks like priority 0 to prevent some router from entering the election from the first place.
after these election some routers neighbors will continue to full state and other will stop here at 2-way.
- In point-to-point router will form full adjacent with the next router .
- In broadcast on non-broadcast a router will form
- full adjacency with the DR and BDR only.
- 2-way with other routers.
that how the DR see other routers:
R1#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 184.108.40.206 1 FULL/BDR 00:00:37 10.10.10.2 FastEthernet0/0 220.127.116.11 1 FULL/DROTHER 00:00:32 10.10.10.3 FastEthernet0/0 18.104.22.168 1 FULL/DROTHER 00:00:37 10.10.10.4 FastEthernet0/0 22.214.171.124 1 FULL/DROTHER 00:00:31 10.10.10.5 FastEthernet0/0
that’s how non-DR see other routers:
R5#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 126.96.36.199 255 FULL/DR 00:00:34 10.10.10.1 FastEthernet0/0 188.8.131.52 1 FULL/BDR 00:00:36 10.10.10.2 FastEthernet0/0 184.108.40.206 1 2WAY/DROTHER 00:00:32 10.10.10.3 FastEthernet0/0 220.127.116.11 1 2WAY/DROTHER 00:00:36 10.10.10.4 FastEthernet0/0
- each router will have full/DR & full/BDR and 2-way with other routers .
- DR/BDR will have full with every one of them.
the next stages will be for routers that is going to the full state. but i wont go to much details about them.
- before the exchange, Master and slave election are made, highest RID is the Master and it start the Exchange and determine the start sequence number
- the DR/BDR can be a slave if they have lower RID, and the priority made them become DR/BDR.
- Exchange the DBD that contain link state advertisement Headers, which describe the whole the database.
Problem : neighbor stuck on Exstart/Exchange
- check MTU mismatch.
7 : loading
- the actual exchange of link state information.
- the router will check the DBD header that he receive during the exchange and will send LSA request for the missing part’s and the respond will be on LSA update .
8 : full
- done .
General information :
- all DBD are resent every 30 minute.
- a route will be deleted after an hour, so it gets a refresh every 30 min.