or: How do I find out which non-cisco systems on my networks generate IP-RIP
   information without letting them mess up my routing tables.
 
Here you could work with a default administrative distance.
Administrative distance is the basis upon which the cisco prefers
routing information of one protocol over another. In this example:

        router rip
        network 192.125.254.0
        distance 255
        distance 120 192.125.254.17     ! list all valid RIP suppliers
        [...]

the value 255 has the implicit meaning of not putting this information
into the routing table. Therefore, setting an administrative distance
of 255 means that all RIP suppliers are by default accepted but their
information is not put into the routing table. The administrative
distance for the router 192.125.244.17 has been reset to the default
(for RIP) of 120, causing its routes to be accepted into the routing table.

Then you can look them up with ``show ip protocols'' and restore the
original administrative distance for the ones you want to fill in the
routing table.

The same results can be acheived with an ip access-list, but with
that, ``show ip protocols'' will only show the valid ones. But often
it is more useful to see which systems were generating routing
information at all.

This trick works for other routing protocols as well, but please select
the proper adminstrative distance (rather than 120) for the protocol
you're using.