It is the thesis of this article that under the federal rules there are basically only two limitations on the use of extrinsic evidence to attack the credibility of a witness.
First, the basic rules of relevance and considerations of judicial efficiency give the judge discretion to exclude evidence of irrelevant or "collateral" matters under Federal Rule of Evidence 401 and 403. Second, Rule 608(b) limits the use of extrinsic evidence of specific acts tending to show that the witness has the character of a liar or a truthteller. The first of these limitations is discretionary while the second is not. Except for these two limitations, federal courts can and do consider extrinsic evidence on a host of matters relating to the credibility of a witness
Kerper, Janeen and MacDonald, Bruce E.
"Federal Rule of Evidence 608(b): A Proposed Revision,"
Akron Law Review: Vol. 22
, Article 2.
Available at: http://ideaexchange.uakron.edu/akronlawreview/vol22/iss3/2