EXPERT RESPONSE
You generally do not determine link latency with SNMP. Round trip times may be reported in some MIBs, but in many cases it is calculated. Priorities and packet overhead can also increase latency times. Latency can be measured as either SNMP latency or Ping latency. In some cases, the SNMP latency number is the time until the next poll and not latency at all. In short, you have to know what that statistic actually means.
In the ping times, you have a better idea of round trip as these packets are not subject to lower priority in some networks. But that would depend on your network settings, etc. Now, that said, if certain links take longer to process the same packet than others, comparatively, you can determine which links are slower. You will be slower if you travel farther (i.e. a longer cable). Bad cables that are "noisy" may contribute to the times as the DSP's may have to do more eliminate or subtract the noise.
The best type of assessment includes both SNMP data as well as testing of the cables that are on ports that show errors. This will let you know if you have improperly installed cables (which WILL cause problems) and the hardware health. You can create a VLAN for your voice devices to get around some of the issues on your devices.
The assessment should also make sure that you have the same OS revision on all core gear and look to be sure that the buffers are not getting full. You will also want to look at your traffic patterns on your switches to be sure that you put your phones on the "least busy" ones. This step may keep you from buying more equipment (just like checking your cables).
In short – your network should be running well, have the bandwidth available and your cable plant should be in good working order.
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