Connected: An Internet Encyclopedia
E.3.1 The Revised Classic Algorithm
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APPENDIX E Additional Next-Hop Selection Algorithms
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E.3 Some Route Lookup Algorithms
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E.3.1 The Revised Classic Algorithm
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:
- Basic match
- Longest match
- Best metric
- 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:
- It is widely implemented.
- 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:
- It does not handle IS-IS or OSPF route classes, and therefore
cannot be used for Integrated IS-IS or OSPF.
- It does not handle TOS or other path attributes.
- 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.
- The proprietary policy mechanisms currently provided by
vendors are often inadequate in complex parts of the
Internet.
- The algorithm has not been written down in any generally
available document or standard. It is, in effect, a part of
the Internet Folklore.
Next: E.3.2 The Variant Router Requirements Algorithm
Connected: An Internet Encyclopedia
E.3.1 The Revised Classic Algorithm