RFC 4591 and RFC 4349

Ignacio,

Some yeras ago, you explained me the next working of the HDLCPWs and FRPWs:

===
a) HDLCPWs (port mode):

             C1                            C2
    [ FR A ]----[ LCCE 1 ]------[ LCCE 2 ]----[ FR B ]

<------ L2TP HDLCoPW ---->

<------------- FR traffic ------->


The above set of devices will behave exactly
the same (bit for bit on cables C1 and C2) as if C1 and C2
were the same physical cable, eg:

    [ FR A ]----..........................----[ FR B ]

<------------- FR traffic ------->

where the series of dots above represents the transparent PW
between LCCE 1 and LCCE 2.

b) FRoPW (DLCI mode):

            C1                            C2
   [ FR A ]----[ LCCE 1 ]------[ LCCE 2 ]----[ FR B ]

<------ L2TP FRoPW ---->
<----> <---->
         FR traffic                   FR traffic

where "FR traffic" here means Q933 exchanges. The actual data traffic
for each PVC is obviously transported end to end (ie, FR A to FR B
and viceversa).

DLCIs in this mode have local significance.
In other words, Q933 exchanges happen between "FR A" and "LCCE 1" in
one end and between "FR B" and "LCCE 2" on the other end.

In this mode, the L2TP cloud behaves as if it were a FR switch.

===


If we consider a traditional back-to-back communication between two FR 
terminals, using FR terminology (obviously, Q.933 only for SVCs, no for 
PVCs):

     DTE - DCE <== Data Network ==> DCE - DTE
<Q.933> <Q.933>
<------------- FR VC ------------->
<DLCI_1>  [  DLCI switching  ] <DLCI_2>
               [ DLCI_1 => DLCI_2 ]

Now, if we use PWs in the left "DTE - DCE":

a) HDLCPWs:

     DTE [ - PE <== HDLCPW ==> PE - ] - DCE <== Data Network ==> DCE - DTE
<-------------Q.933--------------> <Q.933>
<------------------------- FR VC ------------------------------->
<------------DLCI_1 (Q.922)------>    [ DLCI switching  ] <DLCI_2>
                                             [ DLCI_1 => DLCI_2]

     So, in this case, DTEs do not notice any difference with the 
tradicional scenario.

b) FRPWs: according with your explanation, I would understand the next one:

                                            ¿ ?
                                             |
                                             |
                                            \ /
     DTE - PE/DCE <== FRPW ==> PE/DCE - ¿¿¿DTE??? <== Data Network ==> 
DCE - DTE
<Q.933> <Q.933> <Q.933>
<-------------------------- FR VC ------------------------------------->
<DLCI_1> [DLCI switching] <DLCI_2>         [ DLCI switching ] <DLCI_3>
               [DLCI_1 => DLCI_2]                  [DLCI_2 => DLCI_3]

     However, it would not be possible, due to the access nodes to the 
data network must be DCEs.

Would you clarify how would be this last scenario?

Regards,

Javi