bridge/cRio communications sporadic

[jhersh]

Originally Posted by Ziaholic

I'm glad to see that NI was able to reproduce it ... but sporadic errors are difficult to troubleshoot.

We've identified what's going on with the wireless bridge (gaming adapter). It seems that it maintains an arp table to direct traffic to its wired interface. It makes sense that it would want to filter what traffic it puts onto its wired interface to limit the wasted bandwidth from traffic destined for devices that are not on that segment of the network. However it only snoops arp traffic, so if the classmate has already cached the MAC address of the cRIO (i.e. you haven't stopped the driver station app on the classmate) then there is no arp request for the wireless bridge to snoop.

The reason it works sometimes is that there is a race condition between the cRIO network interface coming up and the bridge connecting. When the cRIO boots and brings up the network interface, it sends a gratuitous arp reply to announce itself. If the bridge is already booted, this gratuitous arp reply will cause the bridge adds the cRIO to its list-of-devices-connected-to-its-wired-interface and begin to put traffic destined for the cRIO on its wired interface (where the cRIO is) and therefore will work. If the bridge is not booted when this gratuitous arp reply is sent, then when it is booted, there is no arp traffic for the bridge to snoop and therefore won't forward traffic destined for the cRIO to its wired interface, making the cRIO unreachable. This very fact is why a warm boot of the cRIO fixes the problem (since there will be a new gratuitous arp reply from the cRIO).

The other way to address the issue is to force the classmate to resend its arp request for the cRIO so that the bridge can snoop it. After this happens, the bridge knows about the cRIO on its wired interface and starts forwarding traffic and everything works. This can be done by running "arp -d" on the classmate to clear the arp cache.

There will be an update to the driver station application that will clear the arp cache if it has trouble connecting to the cRIO.

Cheers!
-Joe