[mrtg] Re: Cisco Switch: Interfaces don't add up right

Steve WOODIN Sjwoodin at gov.pe.ca
Thu Apr 7 15:36:13 MEST 2005

>>> Shane Presley <shane.presley at gmail.com> 4/7/2005 9:46:17 AM >>>
When we fire off our backups at midnight, we see lots of ports max out
(95% utilization or about 11MB/s).  They all are sending data to our
uplink port (media server).  But what's odd is the gigabit uplink port
only shows 10% utilization (11MB/s).

Since we have multiple ports sending data at exactly the same time
(midnight) and MRTG accurately reflects this, we expected to see an
aggregate bandwidth to the uplink of about 40MB/s.  If you look at the
graphs and add up the various servers sending 11MB/s in, how can the
uplink also only be doing 11MB/s out.<<<

Umm, because it's a switch and not a hub?  Remember the way switching
technology operates.  It tries to preserve the bandwidth and keep each
connection "private" and not share it like a hub would. Therefore you do
not "add up" the total data going in, because the switch, if at all
possible, is going to try to preserve the speed for each connection. 

So in your case:

Server 1 - Media Server = 11Mbps
Server 2 - Media Server = 11Mbps

Uplink shows 11Mbps

and if you had a setup like this:

Server 1 - Media Server = 100Mbps
Server 2 - Media Server = 11Mbps

Uplink shows: 100Mbps

Now in your case if you had 4 servers connected via a hub,and the hub
was connected to a switch port and then had that sending data to the
uplink port you get 4X11mbps to the uplink.

The actual numbers on the gigabit port going out may fluctuate a litte
bit but should remain close to the largest amount of data it is

Essentially your servers are not even coming close to taxing your
gigabit port, it is easily handling what you are throwing at it. (this
is a good thing :P)

There may be another way to make MRTG realize what you want which from
what you are saying you want the TOTAL amount of data going across it, I
myself am not sure, perhaps some others on this list have some
suggestions?  (Try Jan's he seems to know what he is talking about. :P


Steve Woodin A+,CNA6
sjwoodin at gmail.com

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