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 ...