E.3.1 The Revised Classic Algorithm

The Revised Classic Algorithm is the form of the traditional algorithm that was discussed in Section [E.1]. The steps of this algorithm are:

1. Basic match
2. Longest match
3. Best metric
4. Policy

Some implementations omit the Policy step, since it is needed only when routes may have metrics that are not comparable (because they were learned from different routing domains).

The advantages of this algorithm are:

1. It is widely implemented.

2. Except for the Policy step (which an implementor can choose to make arbitrarily complex) the algorithm is simple both to understand and to implement.

Its disadvantages are:

1. It does not handle IS-IS or OSPF route classes, and therefore cannot be used for Integrated IS-IS or OSPF.

2. It does not handle TOS or other path attributes.

3. The policy mechanisms are not standardized in any way, and are therefore are often implementation-specific. This causes extra work for implementors (who must invent appropriate policy mechanisms) and for users (who must learn how to use the mechanisms. This lack of a standardized mechanism also makes it difficult to build consistent configurations for routers from different vendors. This presents a significant practical deterrent to multi-vendor interoperability.

4. The proprietary policy mechanisms currently provided by vendors are often inadequate in complex parts of the Internet.

5. The algorithm has not been written down in any generally available document or standard. It is, in effect, a part of the Internet Folklore.