> Source: [sk74700](https://support.checkpoint.com/results/sk/sk74700)

# sk74700 - Unable to establish OSPF Neighbor State

| Property | Value |
|----------|-------|
| Solution ID | sk74700 |
| Date Created | 2012-05-24 |
| Last Modified | 2017-10-24 |
| Technical Level | Advanced |
| Products | Security Gateway |
| Versions | R82.10, R82, R81.20 |
| OS | Gaia |

## 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

This solution requires authentication. Please log in to view the full solution.

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