# Interfaces

The term interfaces is used in a very specific way in the JunosE CLI and this documentation. Interfaces are both physical and logical channels on the router that define how data is transmitted to and received from lower layers in the protocol stack. Conceptually, you configure an interface as part of the physical layer, layer 1.

For example, you can configure the physical and logical characteristics of T3 and T1 lines coming directly from the customer premises or from a central office switch and OC3 lines going out to the core of your network infrastructure. These physical and logical characteristics define an interface.

Interface layering must always be configured in order from the lowest layer to the highest layer. For example, if you have already configured IP to run over ATM and you want to reconfigure the interface to run IP over PPP over ATM, you must first remove the IP interface, apply PPP, and then reapply IP.

## Subinterfaces

A subinterface is a mechanism that allows a single physical interface to support multiple logical interfaces or networks. Several logical interfaces or networks can be associated with a single physical interface. Configuring multiple virtual interfaces, or subinterfaces, on a single physical interface allows greater flexibility and connectivity on the network.

Protocols such as Frame Relay and ATM require that you create one or more virtual circuits over which your data traffic is transmitted to higher layers in the protocol stack. The router requires that you define a subinterface on top of a physical interface as a platform for a virtual circuit, such as a permanent virtual circuit (PVC).

Once you have defined the underlying characteristics of an interface, use the interface command to:

1. Assign an interface type, such as POS or ATM.
2. Assign the associated interface specifier to the interface, such as the slot/port or slot/adapter/port and channel/subchannel.
3. Assign one or more subinterfaces.

## interface Command

The interface command has the following format:

interface interfaceType interfaceSpecifier

Each interface type has an interface specifier associated with it. The interface specifier identifies the physical location of the interface on the router, such as the chassis slot and port number, and logical interface information, such as a T1 channel on a channelized T3 interface.

For detailed information about interface types and specifiers and for specific syntax for the interface command, see the Interface Types and Specifiers in JunosE Command Reference Guide.