Routing is one of the few places where the Robustness Principle (be liberal in what you accept) does not apply. Routers should be relatively suspicious in accepting routing data from other routing systems.
A router SHOULD provide the ability to rank routing information sources from most trustworthy to least trustworthy and to accept routing information about any particular destination from the most trustworthy sources first. This was implicit in the original core/stub autonomous system routing model using EGP and various interior routing protocols. It is even more important with the demise of a central, trusted core.
A router SHOULD provide a mechanism to filter out obviously invalid routes (such as those for net 127).
Routers MUST NOT by default redistribute routing data they do not themselves use, trust or otherwise consider valid. In rare cases, it may be necessary to redistribute suspicious information, but this should only happen under direct intercession by some human agency.
Routers must be at least a little paranoid about accepting routing data from anyone, and must be especially careful when they distribute routing information provided to them by another party. See below for specific guidelines.