The MPLS-OPS Archive

["Gibson, Mark" <mgibson@orchestream.com>]

I'm trying to understand what the resulting packet format of Cisco's AAL5
AToM feature looks like on the MPLS tunnel between the ingress and egress
routers.  I'm working from "MPLS AToM - ATM AAL5 over MPLS" (either of the
IOS12.0(21)ST or 12.0(22)S versions).

I think what's happening is this:

1) LDP distributes Labels for the /32 route to the destination LSR (the
endpoint of the AAL5 tunnel) for use as a transport label.

2) There is some LDP peering that includes the tunnel key (32-bit field in a
doubtless proprietary TLV) that propagates back a label that is used as an
inner label (a la Martini).  The tunnel key must be the same at both ends. 

3)  The tunnel key is used to fill in the ATM forwarding mapping with the
incoming VPI = assigned label, incoming VCI = cli entered value.  This VCI
value is the same at both ends.   the tunnel key thus binds together the cli
config entered at both ends of the tunnel. 

Therefore AAL5 PDUs on a specified incoming VCI/VPI at the tunnel head end
are mapped onto a 2-label stack, the outer = transport label to /32 address,
the inner = the label returned from the tunnel destination.  

Outer label swapped across the LSP and php'd. The inner label is read at the
tunnel destination LSR and used to index the ATM forwarding table where the
correct forwarding VPI/VCI values are determined and the PDU is Segmented
and forwarded in the appropriately labelled cells. 

Have I got this right?  I can obviously configure this service based on the
documentation but its driving me mad not understanding how this works.  Off
line responses are fine. 

Cheers

Mark

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