Configure the Management and Loopback Interfaces

The major characteristic of network devices is that they have network interfaces, and usually more than one. Routers can have literally hundreds and so can large switches. The Junos OS supports many types of interfaces, but only two do not support user traffic — the management interface and the loopback interface.

Configure the management interface

One interface you need to set up on the router is the router’s management interface, which is called fxp0 on many Junos OS devices. You use this interface for out-of-band (meaning “not over the same network the user data travels”) access to the device.

Unlike other network interfaces on the device, which receive and transmit traffic flowing between different network interfaces on the device (transit traffic), the out-of-band management interface accepts traffic only to and from the router itself.

Using a separate, dedicated interface for managing the router is good for two reasons:

• It doesn’t interfere with network traffic.

  
  



• The interface is available even if other network interfaces go down.

  
  

The fxp0 interface is an Ethernet interface running IPv4, so you configure it like this:

[edit interfaces]
user@junos-device# set fxp0 unit 0 family inet address 192.168.50.2/24

Here, the default physical interface values for the fxp interface type are accepted and assigned the IPv4 address (family inet), a logical property, at the unit 0 level. And, yes, there can be other units with other addresses, or even more than one address on unit 0!

Configure the loopback interface

Oddly, the device’s loopback address is not used to loop traffic back to the sender. However, it is a stable address for the device that can be used whether or not a particular physical interface is down. Once each network interface has an address, it can send and receive traffic.

However, if the interface goes down or if you need to remove it from the router for some reason, the router may no longer be reachable on the network by applications such as SNMP that monitor the router. To prevent this issue, you assign an address to the router by configuring the router’s loopback interface:

[edit interfaces]
user@junos-device# set lo0 unit 0 family inet address 192.168.10.1/32

The loopback address is a special internal interface within the router and is not associated with any physical hardware, so you don’t need to specify an FPC or PIC slot or a port number.

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Source:http://www.dummies.com/how-to/content/configure-the-management-and-loopback-interfaces.html