[CentOS] CentOS/SNMP update breaks MRTG?

I got itchy fingers over the weekend and decided to fix what wasn't
broken and upgraded one of the older servers from Centos 5.2 to Centos
5.3. Following the recommended process of updating glibc and such
before the rest, it appeared to work perfectly and rebooted without
problem.

However, MRTG 2.15.2 started complaining about unexpected values. I
installed/updated both MRTG (2.16.2) and net-snmp to the latest
available in hope of fixing it. Subsequently, MRTG stopped working
altogether.

I've spent the whole weekend and whole Monday morning trying to fix it
and thus far have only finally managed to get garbage values showing
up in MRTG again as opposed to nothing. And this required learning
about SNMP and adding many additional lines to  the original MRTG
configuration file, none of which I had to do previously.

Did anybody else have similar experiences with MRTG failing after the
update and what was the simple fix? It does not make any sense that I
have to jump through so much hoops to get just the default
functionality back. Thus I believe there must be one small thing I'm
overlooking.

Thanks for any advice.