The difference between AToM and L2TP is that AToM is over an MPLS network whereas L2TP is over straight IP.
The concepts are very similar. The CE facing interfaces on the PEs need to create a VC across IP (for L2TP or MPLS for AToM). This requires pseudowires to make that connection.
With both AToM, the use of LDP neighbor/peer of the remote PE is used but in L2TP, under the pseudowire-class, you need to define a ip local inteface loop 0. And then use the peer's loopback as the xconnect's peer.
Config:
R3 - PE
l2tp-class L2TP
authentication
password 7 047822352C0E
!
pseudowire-class L2TP
encapsulation l2tpv3
protocol l2tpv3 L2TP
ip local interface Loopback0
!
!
interface FastEthernet1/1
no ip address
duplex auto
speed auto
no cdp enable
no clns route-cache
xconnect 4.4.4.4 12 pw-class L2TP
