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Under which circumstances Cisco IOS interface SNMP counters(for example ifHCOutOctets or ifInErrors) start from zero? I am aware that clear counters does not affect SNMP counters. In addition, as far as I know, it is not possible to clear SNMP counters from CLI or with snmpset. However, reloading the router will. In addition, counter wrap(for example ifOutOctets will get larger than 4294967295) will cause SNMP counters to start from zero. Are there any other circumstances where Cisco IOS interface SNMP counters can start from zero?

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SNMP counters will not be cleared while the device is up. The idea is that the software polling the counters will keep a tally of the current counter and simply looks at the delta since the last poll. See also this Cisco FAQ about SNMP counters, which says:

Q. How do I poll queue limit drops on a router?

A. With the use of SNMP, there is no way for the show interfaces command to break out the individual elements that go into the output drops.

Consider this new information about what goes into the output drops counter:

Input drops = Queue limit drops + Throttling drops + SMT queue full drops + RSRB drops + no buffer drops

In addition, SNMP counters are never cleared, even if the interfaces are cleared.

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    I don't have time myself, but you might check whether counters reset on a linecard intf if you OIR a linecard in a modular switch / router (such as the Cat6500) – Mike Pennington Jun 5 '15 at 13:53
  • @MikePennington Good point! I tested this out myself on WS-C4506 switch and removal/insertion of WS-X4306-GB and WS-X4448-GB-RJ45 causes both CLI and SNMP counters to reset. So in a nutshell, interface SNMP counters can reset in case of OIR, device reload or counter wrap? – Martin Jun 5 '15 at 15:16
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    I can't test it right now but I think the counters for "deletable" interfaces (Like Vlan, Port-Channel, ...) would reset to 0 when the interface is deleted and recreated. One might argue that this is not the same interface, just the same name. :) – Sebastian Wiesinger Jun 6 '15 at 11:21
  • @sebastian, you have a good point because someone might actually delete an interface as part of a maintenance window procedure – Mike Pennington Jun 6 '15 at 14:45
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    Deleted sub-interfaces are no longer pollable over SNMP. Tested this on Cisco 7206VXR. @Sebastian If I deleted VLAN interface on Catalyst family switch and later recreated the VLAN interface, then both CLI and SNMP counters were preserved. In case of port-channels, the CLI counters were preserved and SNMP counters were zeroed in 3560-X family switch when I deleted and recreated a port-channel. However, after same operation in 4500 family the CLI counters were zeroed and SNMP counters were preserved! So looks like this behavior is platform/software dependent. – Martin Jun 11 '15 at 10:25

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