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Agent-based measurement can fail for a number of reasons, some of them beyond the control of the CNA system.
You should be sure your targets can be reached through your edge routers.
Check to be sure that all required static routes are defined on the routers, tunnels between the USTATs and edge routers are configured properly, and your PBR rules on the edge routers permit traffic out of the USTATs and back in to the appropriate VIP addresses.
You can use the CNA CLI commands traceroute and tcpdump to discover where your probes are going. The CNA implementation of the ping command can tell you if your targets can be reached. Use the USTAT module implementation of these commands when debugging measurement problems.
In addition, two show commands, show active-measurements and show statistics, provide detailed information about your active probe configurations.
A series of closely spaced back-to-back DNS lookup failures can cause the CNA am-scheduler process to fail to carry out its primary mission of sending out probes.
If this happens, the CNA log will contain a series of info-level messages. Use the command debug am-scheduler to see them.
You will also need to track down and fix the reason your DNS server is slow to respond, or else the condition will reoccur. If you are certain you are not sending too many DNS-dependent probes too fast, you should verify that the host names in your CNA configuration are valid and that your DNS server is properly configured. You may have to restart your DNS server if it is running an early version of BIND that is prone to memory leaks.
Output of the show statistics am-scheduler command includes a table of DNS average lookup times plus an estimate of how many DNS lookups per second will be required to accommodate your configured probing rates.
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