Measuring T1 usagewith MRTG

[Matt Shields]

Keep in mind this is only true if you have a Point-to-Point T1
(1.54mbps).   Now days most ISPs are selling T1 ATM circuits and tell
you it's a full 1.54mbps.  ATM has a greater overhead and you usually
get between 1.2 to 1.3mbps.  But the advantage of the ATM circuits is
you can easily bond up to either 6 or 8 (can't remember off the top of
my head) very easily.  You don't need to mess with multiple routes for
each T1 or BGP routing, the ATM bonds at Layer 2 (Data Link) instead of
Layer 3 (Network) like BGP or static routes or any other routing
protocol.

Matthew Shields
Sr Systems Administrator
NameMedia, Inc.
(P) 781-839-2828
mshields at namemedia.com
http://www.namemedia.com
 
-----Original Message-----
From: discuss-bounces at blu.org [mailto:discuss-bounces at blu.org] On Behalf
Of dsr at tao.merseine.nu
Sent: Thursday, August 31, 2006 7:11 PM
To: John Abreau
Cc: discuss at blu.org
Subject: Re: Measuring T1 usagewith MRTG

On Thu, Aug 31, 2006 at 05:53:21PM -0400, John Abreau wrote:
> In the meantime, I'm assuming that the statistics for the switch port
> that the router is connected to is an accurate reflection of the
> traffic through the router. MRTG is showing the following:
> 
>     Max  In: 778.9 kb/s (7.8%)
>     Average  In: 92.5 kb/s (0.9%)
>     Current  In:  	81.6 kb/s (0.8%)
> 
>     Max  Out: 1200.8 kb/s (12.0%)
>     Average  Out: 74.1 kb/s (0.7%)
>     Current  Out: 37.6 kb/s (0.4%)
> 
> We have a single T1 connection, which provides 1.44 Mbps of bandwidth.
> If I'm interpreting this correctly, we're currently using about
> 26% of the T1's bandwidth and averaging 51%, with today's peak usage
> coming in at 83% of the T1's capacity.

Nope. You are averaging 92.5/1500 Kb/s, which is 1/15 of the T1
capacity; your peak is 1200/1500, about 80%.

A T1 is bidirectional; in and out are not related.

> Am I interpreting this correctly? Also, I imagine there's a certain
> amount of overhead in a T1, and I'm not sure how much that would be.

Same IP packet overhead as anything else. Think of it as a
point-to-point serial link.

> Would the figure above of 1200 Kbps Max Outbound indicate that our T1
> was completely saturated at that point?

No, just 80%.

-dsr-


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