New prefix lengths in router renumbering

[David Malone <dwmalone@maths.tcd.ie>]

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I've been browsing the RFC 2894, in an attempt to understand router
renumbering. The RFC describes how the bits in the new prefix are
formed from the UsePrefix and the existing prefix by taking UseLen
bits from the UsePrefix and then taking the corrisponding KeepLen
bits from the existing prefix.

However, it doesn't seem to be explicitly stated how to calculate
the length of the newly formed prefix. There seem to be two obvious
ways - first to use (UseLen+KeepLen) and the second to copy the
length from the original prefix.

My only hint as to which one is intended is this comment at the
end of example 9.2, where UseLen = 64 and KeepLen = 0:

	Any temptation to set KeepLen = 64 in the second Use-Prefix
	Part above should be resisted, as it would instruct the
	router to sidestep address configuration.

I'm guessing that if UseLen+KeepLen = 128, we get a prefix too
long to be used in address configuration?

So, my two questions are, how should the new prefix length be
calculated and is my understanding of example 9.2 correct?

	David.