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Mikrotik -- Internet
ServerA and ServerB

I'm trying to troubleshoot a problem with a new Cisco ASA 5505. The network design is as above - the Microtik is the existing router, ServerA and ServerB used to plug directly into it.

ServerA has IP, ServerB has IP The ASA is configured with no NAT, a "allow anything" firewall, and uses the microtik as its default gateway. In effect, it is currently a simple IP router; the firewall and VPN stuff will all come later once the basics are working.

Th problem is access to ServerA and ServerB is erratic - sometimes it will work, sometimes it will fail. It can fail for either one of the servers only, or both.

When it is working: The Mikrotik logs show ping packets being sent out over the proper interface The ASA logs show the incoming connections.

When it is failing: The Mikrotik logs show ping packets being sent out over the proper interface The ASA logs show nothing reaching the ASA.

This can fail for one server only (e.g.: the Mikrotik is putting out packets to and, but the ASA is only seeing packets arrive destined for It can fail for one source only (e.g.: ClientA on the users network can ping, but clientB cannot) The problem can also be seen from the mikrotik router itself; sometimes it can ping ServerA and ServerB, sometimes it can only ping one of them

What could be causing this? I can't think of any possible cause that is intermittent and could explain why the problem may occur for one destination server and not others.

edit: Link to ASA config

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I wonder if you need to put in identity NAT configs on the ASA:

nat (inside) 0 0 0
nat (dmz) 0 0 0

Also, check the ARP tables on the ASA (show arp) and the servers to make sure they have the correct IP->hw addresses.

And check the routing tables on every device as well.

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no NAT, checking the ARP table is a good idea. I've managed to determine that the problem only occurs if both servers are connected, so I'll repost a simplified version of the question with new information. – DrStalker Sep 23 '09 at 2:11
up vote 0 down vote accepted

This was eventually traced to a problem in the Mikrotik router; when we upgraded the firmware it resolved this problem.

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