What can cause two OSPF neighbors to be stuck in the EXSTART state?
A.
There is a low bandwidth connection between neighbors.
B.
The neighbors have different MTU settings.
C.
The OSPF interfaces are in a passive state.
D.
There is only layer one connectivity between neighbors.
Neighbors Stuck in Exstart/Exchange State
The problem occurs when the maximum transmission unit (MTU) settings for neighboring router interfaces don’t match. The problem occurs most frequently when attempting to run OSPF between a Cisco router and another vendor’s router.
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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.
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OSPF neighbors are stuck in exstart and exchange state due to MTU mismatch
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XStart – MTU
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