At 2:12 PM -0500 2/20/11, Russ White wrote:
> "... the "semantic of BGP" has never been that the AS Path is used
for anything other than determining if the path is loop free."
That assertion seems to ignore the fact that routing decisions often
take into account path length. Are you saying that such decisions are
not part of the semantics of BGP, as interpreted by most ASes?
The primary purpose of the AS list in the AS Path is to determine the
loop-freeness of the path, much like a metric in an IGP is used for the
same reason. The AS Path length was added to the decision process,
AFAIK, in order to inject the idea of the "least cost," or shortest
path, once you've arrived at a set of loop free paths.
Whatever the primary purpose may have been initially, the fact is
that path length is an important input to path selection as
implemented. That justifies
an effort to ensure that the path seen by a recipient is authentic.
Steve
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