yet more on distance vectors

[Martha Steenstrup <[e-mail]>]

Hi Tony,

I want to make sure that I understand you.  You say that BGP handles
this case as you described in the previous message.  Your previous
message says that:

"When a route is advertised, it is implicitly a replacement of the
previous known route.  Thus, when Y advertises X ..  Y .. X .. Z back
to X, X is obligated to remove X .. Y .. Z."

My question is why?  I thought that BGP would reject X .. Y .. X .. Z
because it contained a known loop (i.e. X appears twice in the route).
But why would BGP also remove the previously acceptable route X .. Y
.. Z?  I don't understand.  I admit that I haven't looked at the BGP
specification in a long time, but I don't remember anything like this
in the specification.  Please point me to the reference.

Thanks,
m