"And what is bad value/threshold value for interface errors/discards?"
For errors, you ideally would like to always have zero percent. However, Ethernet does have an acceptable, by design, bit error rate (which I don't recall what it is, but is a very small).
For discards, again, ideally you like to see a zero value. However, very generally speaking, up to 1% is often considered acceptable, but in some cases that can be too high (even much too high). Further, discards tend to happen in bursts, so a short term drop rate might be a much higher percentage, which is often not "visible" over a longer term average (often 5 minutes).
What's an acceptable discard rate for one kind of traffic might be unacceptable for another kind of traffic. Further, discards impact can vary based on the overall RTT of TCP traffic.
