Funny you say that, because I encountered the same "problem" OP did, and thought like you. However, I have a few switches where the snmpEngineId counter seems to be reset when sysUpTime rolls over. Can anyone confirm this?
IOS (tm) C2950 Software (C2950-I6K2L2Q4-M), Version 12.1(22)EA10, RELEASE SOFTWARE (fc2)
"sh version | inc uptime" gives "uptime is 1 year, 19 weeks, 6 days, 4 hours, 9 minutes", however:
I know that this is an old post but I have a question about this with regards to the iflastchange timestamps. I have multiple 4506's on the network where the sysuptime counters have rolled over multiple times, this is causing a bit of frustration when trying to understand how long an interface has been dormant for. What I would like to know is when an interface state is mark as changed on a device whos sysuptime counter has rolled over multiple times, does the time stamp taken represent:
a time stamp, in ticks since the beginning of the last roll over
does it represent a time stamp in ticks since the last reload (Power up)?
I'm talking raw data here, time ticks polled directly from the iflastchange table.
The reason I ask is that if the first is true then consider this:
If after 5 minutes from when the device powers up interface gig1/1's state is marked as changed. From that point onwards the interface state never changes. Now imagine that it's 5 minutes after the 1st roll over and interface G2/2's state changes. Both interfaces Gig 1/1 and Gig2/2's iflastchange time tick time stamp will show 5 min but only Gig1/1 will be accurate. In this case usuang the SNMPEngine time is useless as the time stamps are taken from a rolled over 32bit counter. I sincerly hope this is not the case.
However if the timestamps are taken based on a none rolled over sysuptime counter like the one we see in the 'sh ver', this would be much more advantageous and would allow for the use of the SNMPEngine time. Considering the same scenario as above if after 1 rollover the time stamps for Gig1/1 should represent 5 min and the time tick time stamp for gig2/2 should represent 496days and 5min. I hope this is the case as it provides more posibilities
when transforming the iflastchange output info.
If you require any further elaboration please let me know
The value represents the value of sysUpTime at the moment the interface last changed state. Your description of two interfaces that show "five minutes" for ifLastChange is correct. To properly detect these kind of rollovers, your NMS will have to remember the values of ifLastChange so long as snmpEngineTime doesn't roll over.
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