OSPF "2WAY" & "ExStart" transitioning question

[EM_Chris]

Hello all!

A ccnp book I was reading was talking about the various STATES from "DOWN" <-----> "FULL"

Just for my own clarification and educational purposes I wanted to capture the packets of a basic OSPF adjacency startup process using two routers connected by Ethernet. This was done by using a simple GNS3 setup and Wireshark.

By looking at the Wireshark capture I can pretty much understand the "DOWN" , "INIT" parts.

but I wanted to make sure I understood when the "2WAY" state was actually finished.

Cisco has this to say about the "2WAY" State

"At the end of this stage, the DR and BDR for broadcast and non-broadcast multiaccess networks are elected."

and

the OSPF RFC 2328 states the following about the "2WAY" State

"The (Backup) Designated Router is selected from the set of neighbors in state 2-Way or greater."

So knowing those facts when looking at these two pictures of the Wireshark capture below would it be correct to says that the "2WAY" state stops at packet 13?

I doubt it could be finished at packet 10 because that packet specifies the DR & BDR as the same IP address

( basically the same router ) I don't see any specific documentation on why it's duplicated in both fields like that.

and if packet 13 is the end of the "2WAY" state then does packet 9 still indicate the start of the "ExStart" State?  

Packet 10

Packet 13

Any answers or thoughts are greatly appreciated! and please let me know if I need to clarify any confusion?

[Giuseppe Larosa]

Hello @EM_Chris ,

Two way state should mean just the following:

R1 sees its own OSPF RID listed in R2 OSPF hello packet and R2 sees R2 OSPF RID listed in R1 OSPF hello meaning that a two way communication between the two routers on the common segment is established.

After all routers in the LAN segment see each other in OSPF Hello packets sent on wire the OSPF DR/BDR election takes place.

At this point each DRother router ( means not DR and not BDR) has to build a full adjacency with DR and BDR using two sessions.

To be noted full state is not reached between two DRother routers they stop the neighbor state machine at the two way state and they are called neighbors and not adjacent. Two way can be the final state between two DRother routers.

For reaching the FULL state a pair of OSPF routers have to elect a Master and this is the Exstart phase followed by Loading where the Master sends the LSA headers in its LS database to the other device ( Loading state).

The other router takes note of all LSAs that are missing in its local db or are older and ask them using LSR Request packets and the other device will provide the full details of the requested LSA in LS Update packets. Actually also the non master device can upload its own LSAs to the Master.

At the end of this process the two routers have the db in sync and reach the FULL state.

Edit:

looking at your packet capture I would say that Two way state ends at packet #8 because packet #9 is already a Database description packet

Hope to help

Giuseppe