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

Shane Presley shane.presley at gmail.com
Thu Apr 7 14:46:17 MEST 2005


We're using MRTG to watch a 26 port Cisco switch. It has 24 100mb
ports, and 2 1000mb ports.  Basically the 100mb ports are connected to
Unix servers, and the 1000mb ports are uplinks connected to our backup
media servers (for tape backup).  So each night our servers back
themselves up by sending data to the uplink port (where the media
server is)

The MRTG configs look right.  It detected the 24 ports as 12.5MB/s and
the uplinks as 125MB/s.

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.

Sorry if that's a little confusion, wasn't sure how to explain it.


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