asterisk users - Jun 2008 - DUNDi question

Hello,

I'm wondering about following  DUNDI setup

Suppose we have 2 Asterisks:  astA and astB  with DUNDI peering active 
between them
and  2  SIP endpoints:   sipA registered with astA and sipB regsitered 
on astB
All this is on the same LAN

now sipA call an number which corresponds to sipB at astB ,  so astA 
lookups thru DUNDI at astB and  forwards the call there.

My question is how this fowarding is done ?
    Using SIP RE-INVITE, or REFER, or using SIP 301 responce with 
Contact pointing at something at astB?
    And does the final RTP stream traverse both Asterisks or only one of 
them or None of them?


Thanks
Vadim