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I use program called PRTG for monitoring Cisco routers. On each Cisco device I made an SNMP setup: snmp-server community public. So it means that NMS which will make an SNMP request to this device must have community string "public" in the request. Without this string it cannot be done nothing.

Next step I started the program and press Auto-Discovery and It got me all my devices with CPU load, traffic load and other characteristics. How the program did this? I did not enter the community string to any program settings.

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Do NOT leave your community string as public. – HostBits May 1 '12 at 12:28
Also, use SNMPv3 if possible. Any modern version of IOS should support it. – James Sneeringer May 1 '12 at 14:25
up vote 5 down vote accepted

Probably because public is the default SNMP community, and the program assumes that most admins prefer insecurity to inconvenience, and so will leave the default community string in place, so it tries that before prompting for a secure community.

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