SA negotiation for alternate address.

hi

1. how can we negotiate an SA for an address different from the source
address of the phase1 IKE packet.

2.this is what my ipsec.conf file looks like
conn sample
      leftid=3ffe::1:207:95ff:fee1:1512
      left=3ffe::1:207:95ff:fee1:1512
      rightid=3ffe::1:207:95ff:fee1:ad50
      right=3ffe::1:207:95ff:fee1:ad50
      ...
can i change the leftid feild to another address , different from the
source address of the packet.?
i.e the one given in left. in my tests it was comparing the leftid with the
source address of the packet and hence i cannot give another address in the
leftid feild.
how is the id feild used . does it update the existing connection
information ?

3. can  using certificate based authentication help,
how will the subject altname feild be used at the responder.?

4. can RSA based public key authentiation method be used, in such a case,

thanks and regards