Interface Properties

Each interface has two types of properties assigned to it: physical properties and logical properties. Physical properties are tied to the entire physical port, whereas logical properties affect only that logical portion of the interface represented by unit numbers or channel numbers.

Physical Properties

A physical property on an interface is any property that should be assigned to the entire physical port. Depending on the interface media, a large range of properties can be configured, but they can be divided into a few major categories:


This aligns the bits as they are transmitted out of the interface. The clocking can be learned either from an external source or from the router itself.


This is the Layer 2 encapsulation that is going to be used on the interface. Examples include Frame Relay, Point-to-Point Protocol (PPP), and Cisco High-Level Data Link Control (HDLC).


This is the maximum transmission unit, which is the maximum size of the frame transmitted from the interface.


These are mechanisms used to verify the operation of the interface. Most encapsulations have keepalives enabled by default, but you can disable them to aid in troubleshooting.

Layer 1/2 options

These are various bit and byte settings for the interface media. For a T1 interface, this includes byte encodings, framing, frame check sequences (FCSs), and line buildouts. In comparison, a Fast Ethernet interface might have options such as flow control, loopbacks, and source address filters.

A physical property should always be configured before any logical identifier, such as a unit number. For example, the following is a serial interface with no logical properties configured but with physical properties of encapsulation cisco-HDLC and no-keepalives, and with clocking set to internal:

se-0/0/2 {
    encapsulation cisco-hdlc;
    serial-options {
        clocking-mode internal;
    unit 0;

Logical Properties

All router interfaces that will send and receive transit traffic require a logical unit to be configured. This logical unit creates a division of the physical interface into multiple parts. For instance, an Ethernet interface can be subdivided into multiple virtual LANs (VLANs), each requiring its own logical unit.


Many router vendors refer to a logical unit as a subinterface; they do not require a subinterface on every physical interface, whereas a Juniper Networks router does.

Some interface types, such as point-to-point interfaces and non-VLAN-tagged Ethernet interfaces, still require a logical unit to be configured. This is a unique feature of Junos and may take a little getting used to if you’re coming from other router vendors’ hardware. These interfaces require a unit number because any logical property that needs to be configured must be defined after the unit number definition. The most common types of logical properties include:

Protocol family

Indicates which Layer 3 protocols can be sent and received on the interface. The router can have one protocol family per logical unit or multiple families per logical unit configured. The most common family configured is family inet, which enables the sending and receiving of all packets in the Transmission Control Protocol/Internet Protocol (TCP/IP) suite (e.g., TCP, User Datagram Protocol [UDP], Internet Control Message Protocol [ICMP], and IP). Other common families are inet6 (IPv6), Multiprotocol Label Switching (MPLS), and ISO (ISIS packets). The families multilink-frame-relay-end-to-end and Multilink PPP are used to create multilink interfaces.

Protocol address

The Layer 3 family address, such as a family inet IP address.

Virtual circuit address

Circuit identifier used when dividing the physical interface into multiple logical interfaces. These could be the VLAN ID, Frame Relay data-link connection identifiers (DLCIs), or ATM Virtual Channel Identifier VCIs).

The logical unit number when configuring VLAN, Frame Relay, or ATM can be any value from 0–16,385. The current best practice, however, is to keep the circuit address the same as the unit number for easier troubleshooting. So, if you have a VLAN ID of 40 configured on your interface, the logical interface should also be a unit of 40, although it’s not required. If you are configuring a point-to-point circuit or non-VLAN-tagged Ethernet, the logical unit number must be zero. Think of this unit as a placeholder for all the logical properties that will need to be configured on that interface.

Here is an example of a T1 interface configuration with the default parameters (PPP encapsulation), family inet support, and an IP address of Note that since this is a point-to-point circuit, the unit number must be configured as unit 0:

t1-2/0/2 {
    unit 0 {
        family inet {

Get Junos Enterprise Routing, 2nd Edition now with O’Reilly online learning.

O’Reilly members experience live online training, plus books, videos, and digital content from 200+ publishers.