how to tell bandwidth utilization

Unanswered Question
May 12th, 2007

Can someone tell me how to tell the bandwidth utilization by doing a "show int"? I know the tx and rx numbers of say 1/255 are there, but Im not sure how to tell the percentage by those numbers. Can someone enlighten me on this please. Thanks.

I have this problem too.
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Manoj Wadhwa Sat, 05/12/2007 - 08:50

If your interface BW is configured to be the actual BW of your link, then a high Tx & Rx load would indicate your link is being highly utilized. If you have any monitoring servers such as concord deployed in your network, you can get the BW in terms of percentage and also a history of the utilization report.

jjoseph01 Sat, 05/12/2007 - 09:21

ok, but I should be able to calculate from those numbers the percentage of utilized bandwidth. Is it the case that if you add both the tx and rx numbers, with a common denominator of 255, you get the total?

Example: tx=100/255 and rx=120/255

If youtake 100 + 120 = 220, then divide by 255, you get .862..., which Im thinking means you are utilizing 86% of the link. Is this the correct way to determine if you are trying to see the bandwidth utilization?

Manoj Wadhwa Sat, 05/12/2007 - 09:43

well, thats a good question .. i am not 100% sure if tx + rx be greater than 255 combined. In fact, i have not observed this keenly to be frank. I would guess the BW utilization to be 220 divide by 510 that would give 43.13%. But probably someone more confident can answer this query.

jjoseph01 Tue, 05/15/2007 - 22:03

All, I have asked TAC this question, and I can not get a simple answer from them on this. Id like to revisit this if anyone has any knowledge on this. Im beginning to think that manoj may be right on this, in that the way to calculate bandwidth utilization on an interface is by saying TXLOAD + RXLOAD / 510 = Bandwidth Utilization. Can anyone verify this?

milan.kulik Tue, 05/15/2007 - 23:27


it has no sense to sum TXLOAD + RXLOAD.

You need to count the inbound and outbound bandwidth utilisation separately.

In your Example: tx=100/255 and rx=120/255 means 39% outbound and 47% inbound bandwidth utilisation.

Of course tx + rx can be greater than 255 combined.

TXLOAD + RXLOAD / 510 would give you something you could call a "mathematical average bandwidth utilisation" but would this value be useful?



jjoseph01 Wed, 05/16/2007 - 00:01

I would think it would be useful in that you would know the utilized bandwidth percentage. Would that not be useful? I understand that tx + rx can be greater than 255, thats why I had the combined "510" in there. So, in your example above, with 39% outbound and 47% inbound, would that be a total bandwidth utilization of 86% ? That would be good to know in my situation. So my question is, is that how you calculate the total used bandwidth?

milan.kulik Wed, 05/16/2007 - 00:20

No, it would be 86% only in a case of half-duplex connection.

As most connections are full-duplex, outbound and inbound bandwidth utilizations are independent.



jjoseph01 Wed, 05/16/2007 - 05:03

I was sitting here thinking about this. I DONT KNOW THIS, IM JUST THINKING OUT LOUD AND LOOKING FOR SOME HELP WITH THIS. It seems to me that no matter if half or full duplex, utilization would still be the same. If half duplex, then one number rises (of the TX or RX) and the other is still 1/255. It seems that with half duplex, you might still add the two together and divide by 255 (TX + RX / 255), but with full duplex, you might add the two together and divide by 510 (TX + RX / 510).

Half duplex might be like 10/255 + 1/255 = 11/255 OR 4% bandwidth utilization.

Full duplex might be like 10/255 + 11/255 = 21/510 OR 4% bandwidth utilization.

Id like to know what you guys think about this, because I somewhat doubt myself on this one. But, it also makes sense to me as well, but again, I could be wrong. Again, Id like your opinion on this.

milan.kulik Thu, 05/17/2007 - 01:44

All I wanted to say:

It has not a good practical sense to "add the two together and divide by 510 (TX + RX / 510)".

In a case of Tx=250/255 and Rx=5/255 you would get 50% bandwidth utilisation. But in fact the line is overloaded! So you need to watch each direction independently.

On the other hand: Tx=120/255 and Rx=125/255 looks OK, but in a case of half-duplex line it's 100% bandwidth utilisation in fact.



lsu_tiger_99 Tue, 06/05/2007 - 14:45

Hi Guys,

I had a very spirited discussion at my office regarding this very subject. For example, let's say we have a DS3 circuit. I do a 'sh int' on that interface and I see the input rate as 25 mbps and output rate as 10 mbps.

In my opinion that translates to 25+10 = 35 mbps, i.e., (35/45)x100 = 77% of BW utilization. My question obviously, is the satement 'Total BW utilization is 77%' correct?




milan.kulik Thu, 06/07/2007 - 03:11


IMHO, DS3 provides 45 mbps for Tx and 45 mbps for Rx at the same time (i.e., full duplex).

So 25 mbps means 25/45=56% input BW utilisation, 10 mbps means 10/45=22% output BW utilisation.

If you want to count "Total BW utilisation" (which doesn't have a practical sense), it would be 35/90=39%.



lsu_tiger_99 Thu, 06/07/2007 - 04:38

Milan thanks for your reply.

I thought that you could not exceed the total bandwidth of 45 mbps (or 1.54 mbps for T1) in any given circuit.

Let me ask the question a bit differently. Is it possible to push 1.5 mbps tx and 1.5 mbps rx over a T1 circuit? I am willing to test this out in a lab!

Also, in my experience I have seen performance issue when the WAN link is consistently above 50% utilized (tx or rx). If I read Cisco or other documents, the concensus is 70%. What is a good percentage to go by?



milan.kulik Thu, 06/07/2007 - 04:49

Hi Tiger,

yes, AFAIK, theory says it should be possible to push 1.5 mbps tx and 1.5 mbps rx over a T1 circuit.

If you can test in a lab, it would be nice.

Yes, that's what I'm tring to say all over this long thread:

The maximum of Tx/Rx bandwidth utilisation is important, not the sum or average!

And a rule of thumb is: If it exceeds 70%, you should start worrying.




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