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

Steve Shipway s.shipway at auckland.ac.nz
Fri Oct 10 00:06:46 CEST 2008

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 querys?

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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