Re: Label Stacking in ATM Cell-mode MPLS

The second label is where the rest of the packet is too :)

There is a L3 packet of, say, 200 bytes. Then you add MPLS tags and
you have 208 bytes. To send via ATM, AAL-5 (atm adaptation layer 5)
does SAR (segmentation and reassembly) with null encapsulation into
48 bytes per cell. This adds some 8 bytes trailer to the frame, so now
216 bytes are sliced into 48 byte chunks.

The topmost label is mapped to a VPI/VCI, and reset to
zero, but stays there in the first cell with its exp bits and
(in this case) non last label bit indication, followed by the second
(VPN) label, and the first 40 bytes of the L3 packet.
Then you have 3 more cells with 48 bytes and last a cell with 16 bytes,
padding and trailer.

You may want to take a look at RFC3035/RFC2684.

If not clear, please ask what is there that you can't explain.

-Carlos

Muhammad Anser Khan @ 5/04/2010 8:24 -0300 dixit:
> Top label is for the PE to PE connectivity. Where is the second VPN
> Label? How can we explain this?
>
> Regards,
> Anser
>
> On Mon, Apr 5, 2010 at 1:19 PM, Carlos G Mendioroz <tron_at_huapi.ba.ar
> <mailto:tron_at_huapi.ba.ar>> wrote:
>
> Topmost label is mapped to VPI/VCI. The rest is just standard (i.e. like
> frame mode) labels. No PHP is done, as the circuit is constructed for
> the whole LSP. (The topmost label is kept in the PDU with a 0 value)
> The stack is not on the ATM *header*, is on the MPLS PDU which SAR lays
> in a sequence of cells.
>
> Is that what you wanted ?
>
> Muhammad Anser Khan @ 5/04/2010 5:31 -0300 dixit:
> > Dear Experts,
> >
> > I want to know how ATM cell supports multiple label in ATM header.
> >
> > For example, In MPLS VPN we need two labels in one packet. How can we
> > achieve this in ATM?
> >
> > I need theoretical answer.
> >
> > Regards,
> > Anser
> >
> >
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>
> --
> Carlos G Mendioroz <tron_at_huapi.ba.ar <mailto:tron_at_huapi.ba.ar>>
> LW7 EQI Argentina
>
>

-- 
Carlos G Mendioroz  <tron_at_huapi.ba.ar>  LW7 EQI  Argentina
Blogs and organic groups at http://www.ccie.net