vRtrTunnelMask indicates the mask to be logical-ANDed with the
destination address before being compared to the value in the
vRtrTunnelDest field. For those systems that do not support arbitrary
subnet masks, an agent constructs the value of the vRtrTunnelMask
by reference to the IP Address Class.
Any assignment (implicit or otherwise) of an instance of this object
to a value x must be rejected if the bitwise logical-AND of x with
the value of the corresponding instance of the ipCidrRouteDest object
is not equal to ipCidrRouteDest.