This ambiguity would only be the case if the ETR has only one RLOC.(Hint - this is a "design implementation" detail that differentiates different LISP proposals, either formally documented or under discussion on the list.)
Only true if the ETR is gleaning a source EID to a source RLOC. But you want to validate the mapping so the ETR sends a Map-Request (with a newly allocated nonce) back to the ITR where the Map-Reply in response has a complete locator-set for the *EID-prefix* (that the original invoking source EID matches to).
This is the mobility feature we prototyped in LISP and are not publicly admitting. ;-)
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