From: [email-masked]
Date: Sat, 27 Apr 2002 17:11:33 +0800

How to deal with MPLS VPN forwarding on ATM LSR ?

   in rfc3035,states "Except in certain circumstances specified below,
     when a labeled packet is transmitted on an LC-ATM interface,
     where the VPI/VCI (or VCID) is interpreted as the top label
     in the label stack, the packet MUST also contain a 'shim header' [3].
     If the packet has a label stack with n entries, it MUST carry a shim
     with n entries.  The actual value of the top label is encoded in the
     VPI/VCI field.  The label value of the top entry in the shim (which
     is just a 'placeholder' entry) MUST be set to 0 upon transmission,
     and MUST be ignored upon reception.  The packet's outgoing TTL, and
     its CoS, are carried in the TTL and CoS fields respectively of the
     top stack entry in the shim."

   So it means that if the packet has a label stack with n entries, the top
   entry in the shim is 0. When I send a VPN packet on an ATM ingress, I
put
   VPN internal label in the shim label stack bottom entry,then out label
is vpi/vci,
   I should put 0 in the shim label stack top entry;

   but this conflict with the following section:

   in RFC3032 states "A value of 0 represents the IPv4 Explicit NULL Label,
     This label value is only legal at the bottom of the label stack"

   What should I do on ATM LSR to support VPN ?