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Unable to establish OSPF Neighbor State
Symptoms
  • Unable to establish OSPF Neighbor State.

  • Member state is flapping between Exch Start to Exchange or stuck at any of these states
    state is Exch Start 4 state changes
    state is Exchange 10 state changes

  • In OSPF trace debug shows:
    OSPF: invalid DD packet: DD MTU is too large (N)
Cause

The most common possible causes of this problem are as follows:

  • Mismatched interface MTU
  • Duplicate router IDs on neighbors
  • Inability to ping across with more than certain MTU size
  • Broken unicast connectivity because of the following:
    • Wrong VC/DLCI mapping in Frame Relay/ATM switch
    • Access list blocking the unicast
    • NAT translating the unicast
  • Network type of point-to-point between PRI and BRI/dialer

Solution
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