According to my knowledge, DBD exchange happens in Exchange state.

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Actually not, see below:

Once the DR and BDR are elected, the actual process of exchanging link state information can start between the routers and their DR and BDR.

In this state, the routers and their DR and BDR establish a master-slave relationship and choose the initial sequence number for adjacency formation. The router with the higher router ID becomes the master and starts the exchange, and as such, is the only router that can increment the sequence number. Note that one would logically conclude that the DR/BDR with the highest router ID will become the master during this process of master-slave relation. Remember that the DR/BDR election might be purely by virtue of a higher priority configured on the router instead of highest router ID. Thus, it is possible that a DR plays the role of slave. And also note that master/slave election is on a per-neighbor basis.

In the exchange state, OSPF routers exchange database descriptor (DBD) packets. Database descriptors contain link-state advertisement (LSA) headers only and describe the contents of the entire link-state database. Each DBD packet has a sequence number which can be incremented only by master which is explicitly acknowledged by slave. Routers also send link-state request packets and link-state update packets (which contain the entire LSA) in this state. The contents of the DBD received are compared to the information contained in the routers link-state database to check if new or more current link-state information is available with the neighbor.

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    Actually,if MTU mismatch occurs,routers will get stuck at Exstart state.According to my knowledge,When DBD are exchanged ,then they will get to know MTU values.But exchanging DBDs happens in Exchange state.So,how come it knows MTU value without exchanging DBDs. – kamakshi Apr 2 at 9:13
  • DBD has nothing to do with MTU. DBD is the database descriptor. MTU is just an attribute like Hello timer. If they are not matching, the DB will not be exchanged, as I know. – user8745622 Apr 2 at 11:09
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    I think ,MTU field exists in DBD packet.So,if DBD exchange happens ,then the routers get to know the MTU values. But if MTU mismatch occurs,router get stuck in Exstart state and actual exchange of DBD happens in Exchange state(which comes after Exstart state) – kamakshi Apr 3 at 5:00

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