OSPF Adjacency Building Process

Ever curious regarding how two routers configured for OSPF become fully adjacent?  The following diagram of the process was modeled directly from RFC 2328, and the steps described gleaned from the Routing TCP/IP Vol I book.  Since we can see mention of a DR, this example must be based on a multi-access network.

1. RT1 becomes active and sends a Hello.  At this point, RT1 hasn’t seen any neighbors, so it reports such and sets its DR and BDR fields to 0.0.0.0.
2. Upon receipt by RT2, RT2 will build a data structure for RT1 and set RT1’s state to Init.  RT2 will then send a Hello packet reporting that it has seen RT1, and will report itself as the DR.
3. RT1 now sees its own RID in the received Hello packet from RT2, so RT1 will now create a data structure for RT2 and set its state to ExStart.  RT1 then begins Master/Slave negotiation with a DD packet with a sequence number of “x”, the Init bit set  to indicate that it is the start of an exchange, the More bit set to indicate that it is not the last DD packet to be sent, and lastly with the MS-bit set to indicate that RT1 wants to be the Master.  No LSA summaries are provided just yet – this is purely for Master/Slave negotiation.
4. Upon receipt of RT1’s DD packet, RT2 sets RT1’s state to ExStart.  RT2 then sends a DD with a sequence number of “y”, and the MS-bit set since RT2 has a higher RID in this example.
5. RT1 agrees that RT2 is the Master and sets its state to Exchange. RT1 informs RT2 of its agreement by sending a DD packet with a sequence number “y” that matches what was sent from RT2 in step 4.  The MS-bit will now be set to 0, and here is where we’ll start to see LSA summaries provided by RT1.
6. RT2 now sets RT1’s state to Exchange as well and sends a DD packet containing LSA headers from its Link State Summary list and increments the DD sequence number to “y+1”.
7. RT1 acknowledges this by sending an acknowledgement with the same sequence number that it received in the DD packet from RT2.  It also begins taking the LSA summaries it received and building a Link State Request list.  This process repeats until RT2 sends the last of its LSA summaries and sets the More bit to 0.
8. Once RT1 receives the last of RT2s LSA summaries and acknowledges it knowing that it has sent the last of its own LSA summaries, the Exchange state process is complete.  Now RT1 will begin processing the LSA summaries it has added to its Link State Request list and transition to the Loading state.
9. Once RT2 receives RT1’s last DD packet, RT2 then sets RT1s’s state to Full because RT2 has no entries in its own Link State Request list.
10. RT1 will send Link State Requests for all entries in its Link State Request list and RT2 will send Link Sate Update packets until RT1’s list is empty.  At that point, it sets RT2 to Full, and the adjacency is complete.