When you connect only the internal interface, you are not allowed to select a Default Gateway for example.
If your One-Arm LB is being consumed from other networks and you need routing you will need to use DG or Static routes (I try to avoid static rules, there is no point of adding complexity)
In a functional point of view, the feature will work with both interfaces the same.
Thanks for your reply, would you mind simplifying it for me what I will need routing, if you have any public topology will help me understand more
Appreciate your help
It is rare when we are talking about one-arm load balancers but maybe you need for example your lb to have dns resolution and your dns is not in the same segment. And others...
But again, is rare in one arm scenarios. Anyway the service works ok with both. (I read long time ago a document saying that you should use internal interfaces for this type of build -i cannot find it now) but in a practical way, I dont find any kind of advantage or disadvantage using up or in interfaces and I've been working with this for a long time now.
Hope that helps!