Why can't I "see" a Numatics device on my network?

By laschrupp ·
On a control network I administer, I have four Numatics solenoids. The Numatics communicate with a Rockwell Allen-Bradley PLC using EtherNet/IP industrial protocol but they also respond to ping packets and have a web interface for setup.

All five of these devices plus an operator HMI computer are connected through a single HP Procurve 2824 switch, which also provides connection to the rest of the plant from this subsystem. This HP switch and all others on this network have spanning tree protocol and non-local multicast traffic disabled.

All the devices are on same subnet, net mask Windows firewall is turned off on all Windows computers and the network is located behind a hardware firewall. This firewall is also the default gateway for all devices on the network.

I desire to monitor the status of the Numatics devices from a computer elsewhere on this network. This computer is running Nagios, using ping and check_tcp to the web interfaces on the Numatics.

The problem I am experiencing is that many of the devices on my network cannot ping or otherwise "see" the Numatics devices. This includes the computer running the Nagios processes. At the same time, other computers can see the Numatics. I see no rhyme or reason to why one works and another does not. These four devices are the only ones on the entire network displaying this behavior.

Here is a simplified layout of my network:

switch-21 (
| -- plc (
| -- hmi (, WinXP)
| -- sol1 (
| -- sol2 (
| -- sol3 (
| \-- sol4 (
switch-7 (
switch-0 (
switch-1 (
| -- dc-1 (, Win2008r2)
| -- dc-2 (, Win2008r2)
| -- collector (, Win2008r2)
| \-- monitor (, Win2012r2)
switch-2 (
\-- desktop (, Win7r2)

All the devices shown above can ping all other devices by DNS host name and IP address except the Numatics sol1 - sol4. Only plc, dc-1, collector and desktop can ping sol1 - sol4. The devices that can see the Numatics are on various switches and the computers are running various versions of Windows. The "closest" computer, on the same switch as the Numatics, cannot see them; a computer five switches away does. One domain controller sees the Numatics; the other does not.

Any ideas for resolving this problem would be greatly appreciated.

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