Routers MUST be able to exchange routing information between separate IP interior routing protocols, if independent IP routing processes can run in the same router. Routers MUST provide some mechanism for avoiding routing loops when routers are configured for bi-directional exchange of routing information between two separate interior routing processes. Routers MUST provide some priority mechanism for choosing routes from independent routing processes. Routers SHOULD provide administrative control of IGP-IGP exchange when used across administrative boundaries.
Routers SHOULD provide some mechanism for translating or transforming metrics on a per network basis. Routers (or routing protocols) MAY allow for global preference of exterior routes imported into an IGP.
Different IGPs use different metrics, requiring some translation technique when introducing information from one protocol into another protocol with a different form of metric. Some IGPs can run multiple instances within the same router or set of routers. In this case metric information can be preserved exactly or translated.
There are at least two techniques for translation between different routing processes. The static (or reachability) approach uses the existence of a route advertisement in one IGP to generate a route advertisement in the other IGP with a given metric. The translation or tabular approach uses the metric in one IGP to create a metric in the other IGP through use of either a function (such as adding a constant) or a table lookup.
Bi-directional exchange of routing information is dangerous without control mechanisms to limit feedback. This is the same problem that distance vector routing protocols must address with the split horizon technique and that EGP addresses with the third-party rule. Routing loops can be avoided explicitly through use of tables or lists of permitted/denied routes or implicitly through use of a split horizon rule, a no-third-party rule, or a route tagging mechanism. Vendors are encouraged to use implicit techniques where possible to make administration easier for network operators.