[mrtg] Error in sum interfaces

[Steve Shipway]

If you are using RRDTool for your backend, then instead of defining
summary targets directly as "Target[c]: a + b", you can avoid many
rollover problems by using routers2 as the frontend and defining a
userdefined summary graph.  To do this, you just define 'Target[a]' and
'Target[b]', then add the routers.cgi*Graph diretives on both.  If you
use the 'total' option you get the total of all component targets.  If
you specify 'nolines' and/or 'nodetails' you can suppress the component
parts and only show the total.  See the routers2 documentation for more
help.

Note that using 32bit counters will always give you a problem if any
individual interface has a throughput greater than approx 109Mbps though
as after this a 32-bit octets counter rolls over twice in a 5-min
window.

Steve (biased advice, I know...)