title: Openmpi setting btl flags --mca btl tcp vs --mca btl_tcp_if_include eth1
body: <p>I am looking the documentation of how to set <code>btl</code> flags to use network specific protocol and specific connection.</p>
  
  <p>I am seeing the <a href="https://www.open-mpi.org/faq/?category=tcp#tcp-selection" rel="nofollow noreferrer">btl selection</a> <a href="https://www.open-mpi.org/doc/v4.0/man1/mpirun.1.php#sect20" rel="nofollow noreferrer">general documentation</a> documentation, but a bit confused. </p>
  
  <p>What I understand (please correct me if I am wrong) If I want to use <code>tcp</code> protocol I need to specify <code>--mca btl tcp</code>. And if I want to use ethernet1 as connection device then I need to specify <code>--mca btl_tcp_if_include eth1</code>. 
  So if I want to use both of these setting I will set both ?
   <code>--mca btl tcp</code> <code>--mca btl_tcp_if_include eth1</code> or just setting <code>--mca btl_tcp_if_include eth1</code> will be okay?</p>