//Accounts controller
[HttpPost]
public ActionResult Login(LoginViewModel model, string returnUrl)
{

    string domain = (string)model.domain
    string userName = (string)model.UserName;
    string password = (string)model.Password;

    try
    {
        DirectoryEntry entry = new DirectoryEntry();
        switch (domain)
        {
            case "OPTION1":
                entry = new DirectoryEntry("LDAP://xx.xx.xx.xx:389", userName, password);
                break;
            case "OPTION2":
                entry = new DirectoryEntry("LDAP://yy.yy.yy.yy:389", userName, password);
                break;

        }
        object nativeObject = entry.NativeObject;
        FormsAuthentication.SetAuthCookie(model.UserName, model.RememberMe);
        if (this.Url.IsLocalUrl(returnUrl) && returnUrl.Length > 1 && returnUrl.StartsWith("/")
                    && !returnUrl.StartsWith("//") && !returnUrl.StartsWith("/\"))
        {
            return this.Redirect(returnUrl);
        }
        return this.RedirectToAction("Index", "Home");


    }
    catch (Exception ex)
    {

        this.ModelState.AddModelError(string.Empty, "The user name or password provided is incorrect.");
    }
    return View(model);
}

//web.config
<authentication mode="Forms">
  <forms name=".ADAuthCookie" loginUrl="~/Account/Login" timeout="60" slidingExpiration="false" protection="All" />
</authentication>


//in AccountViewModels I have
using System.ComponentModel.DataAnnotations;

public class LoginViewModel
{
[Required]
[Display(Name = "User name")]
public string UserName { get; set; }

[Required]
[DataType(DataType.Password)]
[Display(Name = "Password")]
public string Password { get; set; }

[Display(Name = "Remember me?")]
public bool RememberMe { get; set; }
}