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How to Configure Interfaces on Junos Devices

Network devices have network interfaces, usually more than one. Routers can have literally hundreds and so can large switches. The Junos OS supports many types of interfaces. To understand the command used to configure a network interface carrying user traffic, it helps to have an overview of the options available in the set interface command in Junos.

One of the ways to explore the options available in set interface is to use the show interfaces terse command:

fred@junos-router# show interfaces terse
Interface                   Admin Link Proto Local              Remote
fe-0/0/0                    up    up
fe-0/0/0.0                  up    up   inet
fe-1/1/0                    up    up
fe-1/1/0.0                  up    up   inet
ge-1/2/0                    up    up
ge-1/2/0.0                  up    up   inet
ge-1/3/0                    up    up
ge-1/3/0.0                  up    up   inet6 3001::2/64
lo0                         up    up
lo0.0                       up    up   inet

Let’s look at the Interface, Proto, Local, and Remote columns to understand what is needed to configure an interface.

Interface types and their abbreviations

All of the interface types supported in Junos have a two-letter text identifier. Some of the most often used supported interface types are:

Interface Type Interface Text Identifier
Encryption Services Es
Fast Ethernet Fe
Gigabit Ethernet Ge
Loopback lo0
Router internal interface for out-of-band management fxp0
Router interface for internal management fxp1
Serial Se
Services for ES and AS PICs Sp
T1 t1

The numbers following the interface type (such as ge-) refer to the chassis slot, processor, and port on which the interface is sending and receiving bits.

The Local column in the output lists the interface addresses. You set the protocol supported on the interface with the family keyword, and the protocol can be one or more of the more common families such as these (this is not an exhaustive list):

  • inet: For IPv4. Specify a 32-bit IPv4 prefix, followed by a slash and the prefix length.

  • inet6: For IPv6. Specify a 64-bit IPv6 prefix, followed by a slash and the prefix length.

  • iso: For interfaces that need to support CLNS, which is the ISO network layer service protocol that is used by IS-IS. You also need to configure one or more addresses on the router’s loopback (lo0) interface, which IS-IS uses for its interface addresses.

  • mpls: For interfaces that need to send and receive Multiprotocol Label Switching traffic. You don’t need to configure an address for this protocol.

The “family” refers to the type of frame content that the (logical) interface must look for when deciding how to process the bits. What looks like an IPv4 packet with an error, for example, may be a perfectly valid MPLS data unit.

You assign an IPv4 address to a Gigabit Ethernet interface, as follows:

user@junos-device# set interfaces ge-1/2/0 unit 0 family inet address

It’s worth seeing how the address displays in the configuration file because this configuration snippet visually shows the different sections of the interface configuration by the layers of indentation:

user@junos-device# show
interfaces {
  ge-1/2/0 {
    unit 0 {
      family inet {

In this example, the IPv4 address is assigned to the first connector on the third processor of the board in the third chassis slot (1/2/0 — computers always start counting with 0, as though the first day of the month were October 0).

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