The 70% "rule of thumb" is in many network design treatises. I know because I put it there. ;-) I put it in the initial Designing Cisco Networks course which grew a life of its own and the "rule" got listed in all sorts of design books after that.
I got it from years of consulting for Network General (makers of the Sniffer). It was a guideline that we used when doing health checks of customer networks. If we saw 70% for a sustained about of time, that was our indication to do some more sniffing to see if the network was perhaps not healthy. A sustained amount of time was generally 10 minutes. The 70% was for WAN, FDDI, Token Ring, and full-duplex Ethernet. The reason for 70% is that if you go beyond that, and a large burst of traffic happens, then it is possible that the burst can't be accommodated and must be queued. You could go beyond 70% if you aren't too concerned about packets getting queued and you don't expect large bursts. The exact number depends on your performance goals and your traffic patterns. On shared Ethernet networks, we got concerned at a lower level, at about 40 or 50 percent, because of collisions on shared networks. Although it completely depends on the number of stations, how often they send, and how large their packets are, at about 40 percent utilization, a typical shared Ethernet network becomes noticeably inefficient and wastes a lot of bandwidth retransmitting frames that collided. This is not a hard-and-fast rule, and I could write pages on why this might not apply on your network, but it's still a reasonable rule of thumb. If you see any documents that present these "rules" as more than just thumb rules, don't believe them. ;-) Priscilla At 01:46 PM 10/3/01, McMasters, Eric wrote: >To all, > I'm monitoring a network and I'm seeing high network utilization >levels on the main (only) network segment. My question is that I've always >heard the rule is 60-70% and your in trouble and can start experiencing >network degradation. Now my question is where can I find this in writing? >I've been searching CCO and other Internet sites for the better part of a >day now and I haven't found anything stating what percentage falls within >the "best practices / industry standard" arena. > >I know that I have read this info in books, but again I think it just states >this as a rule of thumb. I know all networks are different and the >saturation number may change depending on how many collisions are occurring >etc., but I'm at a loss trying to find this info anywhere. Any assistance >would be greatly appreciated!!! Thanks and I hope everyone has a great >day!! > >Eric ________________________ Priscilla Oppenheimer http://www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=21901&t=21884 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
