[mrtg] Bandwidth monitoring of Cisco 3550 VLANs

Gilbert T. Gutierrez, Jr. mailing-lists at phoenixinternet.net
Tue Sep 9 20:47:53 CEST 2008

I am trying to monitor the bandwidth of some VLANs on a Cisco CAT3550EMI 
switch.  I am terminating the VLANs L3 on this switch but am seeing no 
traffic on reports I am generating.  I know I have about 50-100kbps worth of 
traffic flowing on the VLAN but I am getting reports of 60bps.  I am 
trunking to outside Layer2 switches so monitoring the physical port is not 
an option.  I have included a message below that I found on the web with the 
same problem.  Is the response on the included message the answer?

Thank you,


----- Original Message ----- 
Sent: Friday, March 19, 2004 2:30 PM
Subject: [mrtg] Cisco 3550/VLAN

I'm sure this question has been asked before, but from the posts
I've seen, no
one seemed to really answer the question. We're running a Cisco 3550
and I'm trying to monitor throughput of a given VLAN. The only thing
that I'm
able to do right now is produce MRTG reports on VLAN showing
extremely small
traffic -- as in 100 or below bits per second. (not kilobit -- BIT)
This is
impossible as the actual port on the switch itself reports 600kbps.
But the
VLAN attached to a given switchport doesn't seem to report that

I even tried changing MRTG to reference the switch's IP of a given
block -- still, no go.

This is starting to drive me nuts.

Anyone run into this, or have any suggestions?!


Intruix Tech, Inc.


In my experience, it seems that Cisco reports traffic on a vlan that
has passed from one vlan to another.  Since all that traffic is on
the same vlan, it doesn't report that bit rate.  I was puzzled by
this at first as well, and never did get an official answer from
Cisco.  However, I can tell you that on my multi-vlan 4006 I get a
large amount of traffic reported on each vlan as a lot of data
traverses back and forth across them all.  But on my edge network
switches, the vlan report shows next to nothing.


Eric Brander
Eric_Mailing_List at rednarb dot com

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