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

2 Answers 2


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.

  • 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, 2009 at 2:11

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

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.