[mrtg] Using hrSystemUptime with RouterUptime

[Steve Brown]

I'm using MRTG to monitor several Linux servers running Net-SNMP.  By
default, MRTG uses the value of sysUpTime (1.3.6.1.2.1.1.3) for the
uptime on the HTML pages.  Trouble is that when using Net-SNP,
sysUpTime is the uptime for snmpd, not the box.  The uptime for the
system is in hrSystemUptime (1.3.6.1.2.1.25.1.1).  I want the HTML
pages to display the value of hrSystemUptime instead of sysUpTime.
Here is my current config:

EnableIPv6: no
WorkDir: /opt/apache/htdocs/mrtg
Options[_]: bits,growright
Target[test]: 2:public at 192.168.1.1
MaxBytes[test]: 12500000

According to the docs, the RouterUptime option should allow me to do
what I want.  However, I cannot get this to work.  If I add the
following to my config:

RouterUptime[test]: public at 192.168.1.1

I get the following error when trying to update graphs:

ERROR: You must specify the location part of the RouterUptime oid for
non unique targets! (test)

I've also tried varying combinations of including a numeric OID, a
textual OID, etc, etc. and they all result in the same error above.

I've been searching the list archives because I would have expected
this issue to have come up in the past.  I found this post
(http://www.mail-archive.com/mrtg@lists.oetiker.ch/msg19565.html) that
addresses the problem, but the referenced code doesn't appear to be in
the main distributions anymore (I'm using 2.16.0).  So either I'm
doing something wrong, or the docs are incomplete on using
RouterUptime.

Help is appreciated.