> The L2 p2p link ends at the BTS (?) , but the L3 subnet 
  > ends at GGSN (?)

=>  Not quite. The 'link', as seen by the IP stack, is p2p
between the host and the GGSN.

  > which is a router. BTS supports multiple p2p links, any of 
  > them can have a
  > mcast listener. So the BTS needs a way to figure out to 
  > which L2 p2p link a
  > mcast packet needs to be forwarded. Without MLD support, 
  > how does the L2
  > switch know there is a mcast listener in the link so it can 
  > copy  a packet
  > to that link ?  The only other way would be to manually 
  > configure the L2
  > switch - not scalable.

=> The BTS (node B) is not a switch and doesn't
even terminate the radio link, so it can't route
traffic directly between hosts. The _only_ node
that can do that is the default router (GGSN). 
You can think of it as any p2p link used today.

Hesham

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