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I have multiple servers deployed behind a single switch. All the servers have their own public IP addresses. Suddenly one of them (say A) cannot be accessed from the outside by any means, e.g. HTTP, SSH, etc. However, I can ssh into some other server (say B) behind the same switch, and then can successfully access HTTP and SSH services on machine A. By ssh into A on B, I can also access services provided by B on A, but from A I cannot access anything outside the switch.

I tried to restart machine A as well as the switch, but nothing good happens. At the same time, there are very very short slots of times that I CAN access A outside the switch, but then CANNOT access again all of a sudden. To me it doesn't make sense at all.

The OS on machine A is Ubuntu 12.04 and connected to the switch via normal Ethernet. I can ping to all the machines behind the switch, but cannot ping anything else (doesn't fail, but hangs). All the processes running on A are as normal as before.

btw, the network monitoring administrators told me that some machine behind the switch has been sending out packages frequently during the last week without receiving that much. That may be related to this failure.

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    Try checking the switch ARP tables.... looks like the MAC address of the server got lost on the switch - which can have many reasons. This would result in the switch not knowing where to send the packets. An alternative would be something (else using the same IP and thus confusing the switch. Could also be a broken switch - something I had here at times. It is NOT a IP / TCP etc. issue per se - that is either a duplicate IP or an Ethernet level issue. – TomTom Oct 17 '13 at 9:16
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check Gateway settings and routing of the Server check local Firewall Settings could also be a MAC Adress overflow on one of the other Switches after a broadcast Storm

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