Secondaries sometimes don't respond to notify

[John Wobus]

Notifies sometimes get lost between our bind 9.3 servers.  What can I 
look for as a cause?

Two secondary servers are showing the problem with the same primary 
server.  When the failure occurs, the primary server logs that it sent 
notifies, then logs 'notify retries exceeded' for the secondary in 
question.  The secondary's log shows nothing.  Zones and secondaries 
affected at any particular instance are random: failure occurs for only 
10-40% of the notifications.  When one secondary fails for a particular 
zone, the other one often succeeds in loading it.  The new zone files 
have updated SOA serial numbers. The failing secondary later transfers 
the zone successfully, when the refresh interval expires.  None of the 
servers have firewall software.  The servers serve fewer than 300 
zones.

I've checked the network, the bind config file options (which are 
generally the defaults), looked for other problems in the logs, 
searched my bind books/manuals and searched online and I have run out 
of ideas.

John Wobus
Cornell CIT