+/** \brief Create or delete a GRE tunnel
+ @param client_index - opaque cookie to identify the sender
+ @param context - sender context, to match reply w/ request
+ @param is_add - Use 1 to create the tunnel, 0 to remove it
+ @param is_ipv6 - Use 0 for IPv4, 1 for IPv6
+ @param tunnel_type - 0: L3, 1: TEB, 2: ERSPAN
+ @param instance - optional unique custom device instance, else ~0.
+ @param src_address - Source IP address
+ @param dst_address - Destination IP address, can be multicast
+ @param outer_fib_id - Encap FIB table ID
+ @param session_id - session for ERSPAN tunnel, range 0-1023
+*/