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Implementing and Operating Cisco Enterprise Network Core Technologies (ENCOR) · Cisco
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use ISIS instead)) D. is right anyway ...
Answer D "Neighbors Stuck in Exstart/Exchange State The problem occurs most frequently when attempting to run OSPF between a Cisco router and another vendor's router. 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." https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/13684-12.html
In this example, there is a mismatch in MTU values between two OSPF neighbors. This router has MTU 1600: OSPF: Rcv DBD from 10.100.1.2 on GigabitEthernet0/1 seq 0x2124 opt 0x52 flag 0x2 len 1452 mtu 2000 state EXSTART OSPF: Nbr 10.100.1.2 has larger interface MTU The other OSPF router has interface MTU 2000: OSPF: Rcv DBD from 10.100.100.1 on GigabitEthernet0/1 seq 0x89E opt 0x52 flag 0x7 len 32 mtu 1600 state EXCHANGE OSPF: Nbr 10.100.100.1 has smaller interface MTU MTU mismatch causes that exact state