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OSPF Neighbors with Virtual Link stuck in Exstart State

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Article ID: KB9886 KB Last Updated: 24 Jun 2010Version: 2.0
Summary:
Virtual Link between Juniper firewall and Cisco 6509 shows up, but OSPF neighbor stops at Exstart state on Juniper firewall and Exchange state on Cisco 6509.
Symptoms:
In the following network topology, an OSPF neighbor relationship was created between the NS-5GT and Cisco using a virtual link. 

Area 10-----NS-5GT----VPN Area 2------ISG-2000-----Area2------Cisco---Area 0

The Cisco was stuck in the Exchange state.
The NS-5GT was stuck in the Ex-start state.
Solution:
In reviewing the packet capture on the Cisco side, the Cisco was sending a DBD packet with the size of 1494 bytes.

However, the NS-5GT had the following ScreenOS command configured:
set zone "Untrust" screen block-frag
Therefore, the DBD packet sent from the Cisco, which was fragmented by the ISG2000, was dropped by the NS-5GT because of the 'block-frag' screen setting.

Disabling the 'block-frag' screen setting on the NS-5GT permitted the fragmented packet ,and then the OSPF neighbor state changed to Full.
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