I have 7513 router to which 20 routers are connected and we are running OSPF within backbone area . So when We do "show ip ospf neighbour" , it shows STATE as "FULL"....but problem arises after 6 or 7 days i.e whenever we execute "show ip ospf neighbors" again , for some neighbors , it shows state as "DOWN" & "EXSTART"......I don't know the reason.....please suggest...
After two OSPF neighboring routers establish bi-directional communication and complete DR/BDR election (on multi-access networks), the routers transition to the exstart state. In this state, the neighboring routers establish a master/slave relationship and determine the initial database descriptor (DBD) sequence number to use when exchanging DBD packets.
This is the first OSPF neighbor state. It means that no information has been received from this neighbor, but Hello packets can still be sent to the neighbor in this state. If a router doesn't receive a hello packet from a neighbor within the RouterDeadInterval time (RouterDeadInterval = 4*HelloInterval by default), then the neighbor state changes from Full to Down.
REASON & SOLUTION:
The problem occurs when the maximum transmission unit (MTU) settings for neighboring router interfaces don't match. If the router with the higher MTU sends a packet larger that the MTU set on the neighboring router, the neighboring router ignores the packet.
Unicast is broken:
In the exstart state, the router sends a unicast packet to the neighbor to elect master and slave.
This is true unless you have a point-to-point link, in which case it sends a multicast packet. The following are possible causes.
a) MTU problem, meaning the routers can only ping a packet of a certain length.
b) Access list is blocking the unicast packet.
c) NAT is running on the router and is translating the unicast packet.
Well, i think most of the cases the exstart state indicates the frequent REELECTION of DR/BDR. It can happen because of a cable connection problem(Flapping),so i think you have check up your connectivity issues(You can do this by looking into your router's interface resets).
A less probable but possible reason would be remote routers forcing an election, like somebody changing the priority etc...
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