Added a route and the behaviour changed - asp.net-mvc

Question

I would like to optimize the appearance of the URL like below:
http://localhost:3817/Affaire/SearchAffaires?OnlyFavorite=True
So I added a new route:
routes.MapRoute(
"Search Affaire Only Favorite", // Route name
"{controller}/{action}/OnlyFavorite", // URL with parameters
new { controller = "Affaire", action = "SearchAffaires", OnlyFavorite = true } // Parameter defaults
);
Now the URL is easier to read:
http://localhost:3817/Affaire/SearchAffaires/OnlyFavorite
But a new problem is occured: the other links on my page have changed because of the routing!
Example here: .../Affaire/SearchAffaires?LabelName=Baxter&OnlyLabel=True&OnlyFavorite=True
Before it was: .../Affaire/SearchAffaires?LabelName=Baxter&OnlyLabel=True
As you can see, the variable OnlyFavorite has been added to the URL. Finally I found the reason of this behaviour: the routing system is keen to make a match against a route, to the extent that it will reuse segment variable values from the incoming URL. The best way to deal with this behavior is to prevent it from happening. It is strongly recommend that you
do not rely on this behaviour, and that you supply values for all of the segment variables in a URL pattern.
That's a little bit annoying because I have a lot of variables!
Any solution on that problem? Why is that behaviour not happening whith one single route (the default one)?
Thanks

Answer 1

It looks like if I've registering routes like this, i get away from problem:
routes.MapRoute(
"Search Affaire", // Route name
"{controller}/{action}", // URL with parameters
new { controller = "Affaire", action = "SearchAffaires" } // Parameter defaults
);
routes.MapRoute(
"Search Affaire Only Favorite", // Route name
"{controller}/{action}/OnlyFavorite", // URL with parameters
new { controller = "Affaire", action = "SearchAffaires", OnlyFavorite = true } // Parameter defaults
);