The extension described in this document allows a pair of PEs
connected via an Ethernet Pseudowire (PW) to signal via the use of
the Control Word (CW) whether the Ethernet frame encapsulated is
coming from a Root AC or a Leaf AC. This allows a PE receiving this
frame to decide whether it can be forwarded to a Leaf AC or not.
Such forward or drop decision is an additional filtering action
after the MAC-based forwarding decision. This applies to both P2P PW
and P2MP PW.