Router Interfaces

Table of contents:


JUNOS routers have three types of interfaces: network, services, and special interfaces. As you might expect, network interfaces physically connect to the network and carry network traffic. Services interfaces manipulate the traffic before transmitting or receiving it, for example, to perform Network Address Translation (NAT), IPSec functions, or monitoring traffic flows. Special interfaces include two internal Ethernet management interfaces and the loopback interface, which is not used for performance monitoring but as a place to define an IP address for the router as a whole. The naming conventions for the three types of interfaces are the same, and you configure them the same way.

For interfaces to work, you must configure them. Simply installing the hardware in the router is not sufficient. The router detects that network hardware is present and you can list the hardware and interfaces with the show chassis hardware and show interfaces terse commands, but they will not carry traffic. You can also configure interfaces that are not present in the router, which is a handy feature when you are preparing to receive new hardware or to move a Flexible PIC Concentrator (FPC) or a Physical Interface Card (PIC) to another slot. When checking the configuration during a commit operation, MGD, the management process (daemon), checks whether the hardware corresponding to the configuration is present in the router. If it is, MGD hands that portion of the configuration over to the proper processes for activation. If the hardware is not present, MGD ignores that portion of the configuration.

When configuring interfaces on the router, you identify the interface by media type and location in the router. The media type is a string, typically two letters, that identifies the network device. Table 7-1 lists some of the common interface media names.

Table 7-1. Some interface media names

Interface type







Fast Ethernet


Gigabit Ethernet









Encryption Services


Generic Route Encapsulation tunnel interface


Monitoring Services


Multicast tunnel interface


Services (for ES and AS PICs)





Out-of-band management


Internal management

The location portion of the interface name identifies which slot the FPC is in, which PIC slot on the FPC the media is installed on, and the specific port on the PIC. Most M-series and T-series routers have either four or eight FPC slots, and each slot has either two or four PIC locations. On J-series routers, there can be up to six FPC slots.

To illustrate interface naming, the interface name for the first port of a Fast Ethernet PIC installed on the FPC in slot 2, in the first PIC position, would be:


For channelized interfaces, such as T1, the interface name includes the channel number. The name for the first channel on a T1 interface would be:


When numbering the slots, ports, and channels in an interface name, the first item is 0, not 1. For routers that have eight FPC slots, the slots are numbered from 0 through 7. Most PICs have four locations, numbered 0 through 3, and port and channel numbering starts at 0. You can find the FPC and PIC slot numbers on the router chassis, and the port numbers on the PIC faceplate.

JUNOS interfaces consist of a number of layers that affect how you configure them. Like an onion with an outer skin and inner layers, the outer skin of the interface is the physical interface, which generally encompasses the entire physical device. On the physical interface, you set properties that control the behavior of the device itself. These properties typically correspond to OSI Reference Model Layer 1 and Layer 2 properties. As examples, Ethernet physical interface properties include the speed (10 Mbps or 100 Mbps) and half-duplex or full-duplex operation; T1 interface properties include framing, encoding, loopback, and setting up a Bit Error Rate Test ( BERT); and for SONET, Automatic Protection Switching (APS) is a physical property. You configure the physical properties of an interface in two places: directly under the name of the interface and in an interface-specific -options section directly under the name of the interface. So for a Fast Ethernet interface to run at 100 Mbps, use the set fe-2/0/0 speed 100m command, which in the configuration looks like this:

router1# show
interfaces {
	 fe-2/0/0 {
	 speed 100m; # <--physical property of the interface

You see that the speed statement is directly under the name of the interface. An example of including the statement in an -options grouping is when setting a SONET interface to run APS. In this case, you use commands that place statements under the sonet-options hierarchy, as in this example configuration:

	aviva@t320# show
	interfaces {
	 so-3/1/0 {
	 sonet-options { # <-- options group of physical interface properties
	 aps {
	 working-circuit APS-at-my-colo;
	 authentication key $1991poPPi;

The next layer of our interface onion is the logical interface (sometimes also referred to as a subinterface), which is the mechanism that divides a single interface into one or more virtual devices. Logical interfaces take all the traffic traversing the physical interface and create separate streams or flows that can have different properties. Virtual device properties include Frame Relay DLCIs and ATM virtual paths (VPs) and virtual circuits (VCs). For a physical interface to function, you must create at least one logical interface on it. Each logical interface is identified by a unit statement under each physical interface and a number that identifies the specific instance. The first logical interface is 0. So, to configure a Frame Relay DLCI, use the set t1-0/0/3 unit 0 dlci 100 command, which results in the following configuration:

	aviva@router1# show
	interfaces {
	 t1-0/0/3 {
	 encapsulation frame-relay; # <-- physical interface property
	 unit 0 { # <-- logical interface opener
	 dlci 100; # <-- logical interface property

When you configure an IGP to run over an interface, the JUNOS software forms IGP adjacencies over the logical interface.

The third layer of the interface onion is the protocol family, which is where you start tying together the routing protocols on the routers and the interfaces on which they can run. For IPv4 protocols to run on an interface, an inet family must be present on the logical interface. Other common families are inet6, for IPv6 protocols, iso for IS-IS, mpls for MPLS, and vpls for VPLS. Multiple protocol families can run on a single logical interface. If you do not configure the appropriate family on the logical interface, the router will not recognize any packets in that protocol family and will discard them even though you have configured the interface in the routing protocol's configuration.

The final layer is the address, which associates the network address with the protocol family and controls other address properties. For any protocol family to operate, an address must be configured. For example, for an IPv4 protocol such as BGP or RIP to work on an interface, you must assign an IPv4 address for the logical interface's inet family with a command such as set interfaces fe-0/0/0 unit 0 family inet address For IS-IS to also run on this logical interface, add the family iso:

	aviva@router1# show
	interfaces {
	 fe-0/0/0 {
	 unit 0 {
	 family inet { # <-- set the IPv4 protocol family
	 address; # <-- set an IPv4 address
	 family iso; # <-- set for IS-IS

This chapter is organized into three groups of recipes. The first group shows how to view interface status. This is something you do regularly to check on the operation of the router and to troubleshoot problems with the interfaces themselves or with the protocol software. The second group of recipes shows how to set various addresses and families on logical interfaces. The final group shows how to configure and troubleshoot specific network and special interfaces.

Router Configuration and File Management

Basic Router Security and Access Control





Router Interfaces

IP Routing

Routing Policy and Firewall Filters







IP Multicast

JUNOS Cookbook
Junos Cookbook (Cookbooks (OReilly))
ISBN: 0596100140
EAN: 2147483647
Year: 2007
Pages: 290
Authors: Aviva Garrett © 2008-2020.
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