OSPF adjacency formation

[chid_1989]

Hi,

I have some basic doubts in basic OSPF adjacency formation.

Assume a topology where in 5 routers R1..R5 are connected by a switch and R3 is the DR and R2 is the BDR. All these routers are fully adjacent. Now another router R6 is connected to LAN.

Can you tell me how ospf adjacency is formed. Or can you clarify these doubts for me.

1. When a hello packet is sent to 224.0.0.5 by R6, all routers in the LAN will receive the hello. Will all the routers reply back to the hello? I am having this doubt since i learnt, the routers only for adjacencies with DR and BDR.

2. After the 2way state, i am confused with the master slave process. R6 will send an empty DD packet. Who will process that DD packet inorder to become the master for the database exchange? Logically i think the DR should, but how does this happen in protocol.

3. Once it crosses the exstart state, on exchanging the lsa's will all the other router be able to listen the lsa's being advertised since it is on a LAN.

Now in another case, assuming this R6 is connected to R1 ( not to the LAN ). R1 is DROTHER. R1 should have established full adjacency with R3 and R2 only. So how will R1 and R6 exchange the database. If R6 happens to be an ABR how will this change.

I think my questions are vague. may be i dont know to put it in proper words. If you are not able to get the question, please tell me.

Thanks

Chidambaram

[Giuseppe Larosa]

Hello Shrihan,

OSPF neighborship comes first when reaching the two way state ( devices see each other OSPF RIDs listed in respective OSPF hellos in segment), devices progress to form  full adjacency with DR and BDR only.

With DR and BDR the neighbor state machine progresses from Two WAY to FULL passing other intermediate states as discussed before in this thread.

Hope to help

Giuseppe

[Alessio Andreoli]

Hi,

i jsu want to add a visual idea of what the above real experts already explained:

Alessio