[mrtg] Can Mrtg cause a Router to crash because of the snmp querys?

Mersberger, Robert robert.mersberger at goldenliving.com
Fri Oct 10 15:49:59 CEST 2008

I Totally agree with Steve and suggest that you put a lot of effort,
quickly, into finding out what product is eating your CPU.  Wireless is
a good suspect especially if you are using GRE.  If you are using Cisco
Wireless products don't let them tell you that your cpu issues are being
cause by snmp.  Have you looked at upgrading your cpu and memory on the
Good luck.


From: mrtg-bounces at lists.oetiker.ch
[mailto:mrtg-bounces at lists.oetiker.ch] On Behalf Of Steve Shipway
Sent: Thursday, October 09, 2008 5:07 PM
To: 'Jack Bauguer'; 'mrtg at lists.oetiker.ch'
Subject: Re: [mrtg] Can Mrtg cause a Router to crash because of the snmp

Our network here is way larger (1500 switches, 60 routers, 1320 hosts),
and we poll not only network traffic but also stats for multicast
traffic and CPU use on the core switches and routers, all at 5min
intervals.  There is no discernable load on the switches from the SNMP
queries.  Of course, our main switches are only on about 20% load, but
remember that these queries are only simple network traffic ones.


Note that, since SNMP is UDP traffic, and low priority to a switch, the
SNMP queries will simply be dropped in favour of TCP traffic if a link
is overloaded and not resent.  You'll get greyout on your MRTG graphs
(if using routers2) or blanks.  Most switches/routers will ignore SNMP
queries in favour of forwarding packets.


If your network is running at 85% loading I'd say you have other more
serious issues in capacity planning to deal with!  If a few SNMP queries
every 5min would cause a network collapse, then you're at far more risk
from (eg) someone opening a web browser...





From: mrtg-bounces at lists.oetiker.ch
[mailto:mrtg-bounces at lists.oetiker.ch] On Behalf Of Jack Bauguer
Sent: Friday, 10 October 2008 02:32
To: mrtg at lists.oetiker.ch
Subject: [mrtg] Can Mrtg cause a Router to crash because of the snmp



Im trying to build a traffic analyzer based on Mrtg here for my company,
(10 big routers 80 switches 80 servers Cisco/Nortel).

There was an argument about creating this Mrtg-Server, some people claim
it would be suicidal, because many of the routers and switches operates
with heavy traffic and cpu load (about 75-85%) and querying these
elements every 5 minutes would collapse the network.

Can somebody give me some feedback on this, I personally dont believe
that querying through snmp every 5 minutes can cause such damage on the
network, even if they are on high capacity, i think maybe it could cause
a small delay on the query but nothing else, right?. I have allready
worked in a smaller enviourment and had no problems, but I need some
technical arguments to convince these guys, 

any ideas?

regards. JB


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