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SNMP is a fairly complex protocol, and the fact that it primarily operates over UDP does not exactly help matters. As a consequence, there are a number of potential problems that affect NMIS' ability to collect information from SNMP agents efficiently and quickly.

snmp_timeout and snmp_retries

By default NMIS has a 5 second SNMP timeout and will retry once before it considers SNMP to have failed.  The default settings work in 99% of circumstances, some devices and/or networks require increased timeout or retries to work better, so these settings can be increased, however it is important to remember that when SNMP is not responding the polling process will now have to wait for the multiple of the timeout and retries, so by default 5 seconds.  If the retries were set to 3 then 5 seconds and 3 retries would be 15 seconds before NMIS considers that SNMP is down.  

For servers with many nodes, it is not recommended for the multiple of timeout and retries to exceed 20 seconds.

snmp_max_msg_size

The primary tunable NMIS configuration setting for SNMP is snmp_max_msg_size, which controls how large a single SNMP packet may be.

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Whenever such an automatic adjustment is attempted, NMIS logs a warning message similar to this example:
"WARNING (servername) SNMP get_table failed with message size exceeded, retrying with maxrepetitions reduced to 36"