Hey AtefKouiri!

The R1-2401 defaults the interfaces to access mode for VLAN 1. This means that the interface will accept all untagged frames, along with VLAN 1 tagged frames. The access mode interface will send only untagged frames. this is commonly used when connecting to edge devices, and switch interconnect on a flat network. In a network where multiple VLANs are in use, switch interconnect would be done through interfaces set to Trunk/General mode.

Here is a quick run down of the different modes.
http://dell.to/1TvJgN4

Let me know if this helps clear things up.