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<strong>Discussion</strong><br />

Most of the IP addresses you configure on a router are for physical network interfaces,<br />

such as an Ethernet or T1 interface. While these interfaces are generally<br />

present and operating in the router, you don’t want to use them to find out the status<br />

of the router, because someone might remove the PIC from the router or because<br />

the physical interface may not be up or may not be configured properly even though<br />

the router is up and running fine. The solution is to configure an IP address that<br />

uniquely identifies your router. You do this by configuring the lo0, or loopback,<br />

address.<br />

In the JUNOS software, the loopback address is named lo0. This name doesn’t follow<br />

the standard JUNOS interface naming conventions. It is just the same name as<br />

the Unix loopback interface. You don’t need to specify a PIC, FPC, or slot number<br />

because the loopback address is not associated with hardware, but is just an internal<br />

address on the router. You configure the loopback address on logical unit 0, and the<br />

address is an IPv4 address (set with the keyword inet). In this recipe, we set the<br />

loopback address to 192.168.16.1/32.<br />

The loopback address on a JUNOS router is used as the IP address of the router itself<br />

and is similar to the Unix localhost, which is the address of the local system. The<br />

JUNOS software uses this address when it needs to talk to itself using IP.<br />

To see that the loopback address has been configured, you can look at its status:<br />

aviva@RouterA> show interfaces lo0<br />

Physical interface: lo0, Enabled, Physical link is Up<br />

Interface index: 6, SNMP ifIndex: 6<br />

Type: Loopback, MTU: Unlimited<br />

Device flags : Present Running Loopback<br />

Interface flags: SNMP-Traps<br />

Link flags : None<br />

Last flapped : Never<br />

Input packets : 19<br />

Output packets: 19<br />

Logical interface lo0.0 (Index 69) (SNMP ifIndex 16)<br />

Flags: SNMP-Traps Encapsulation: Unspecified<br />

Input packets : 0<br />

Output packets: 0<br />

Protocol inet, MTU: Unlimited<br />

Flags: None<br />

Addresses, Flags: Is-Default Is-Primary<br />

Local: 192.168.16.1<br />

Logical interface lo0.16385 (Index 64) (SNMP ifIndex 21)<br />

Flags: SNMP-Traps Encapsulation: Unspecified<br />

202 | Chapter 7: Router Interfaces<br />

This is the Title of the Book, eMatter Edition<br />

Copyright © 2008 O’Reilly & Associates, Inc. All rights reserved.

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