Cacti – monitoring intervals and ~114Mbps

If you are using Cacti, and find that the graphs for an interface appear to have a maximum of around 114 Mbps (on a >=200Mbps interface), check:

  • 32-bit or 64-bit counters (default 32-bit)
  • Check interval (default 5min)

With a 5min interval, the 32-bit counter will likely overflow. Need to switch to 64-bit counters (if supported by the device) and/or reduce the interval (this is more complicated and may mean you loose history).

The Cacti defaults work against the >200Mbps interface, which is a common case.

From the Cisco SNMP Counter FAQ, this explains why:

Q. When should 64-bit counters be used?

A. RFC 2233 adopted expanded 64-bit counters for high capacity interfaces in which 32-bit counters do not provide enough capacity and wrap too fast.
As the speed of network media increases, the minimum time in which a 32-bit counter wraps decreases. For example, a 10 Mbps stream of back-to-back, full-size packets causes ifInOctets to wrap in just over 57 minutes. At 100 Mbps, the minimum wrap time is 5.7 minutes, and at 1 Gbps, the minimum is 34 seconds.

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