Configure the Primary Interface for the Router

The primary interface for the router has the following characteristics:

• It is the interface that packets go out when you type a command such as ping 255.255.255.255—that is, a command that does not include an interface name (there is no interface type-0/0/0.0 qualifier) and where the destination address does not imply any particular outgoing interface.
• It is the interface on which multicast applications running locally on the router, such as SAP, do group joins by default.
• It is the interface from which the default local address is derived for packets sourced out an unnumbered interface if there are no non-127 addresses configured on the loopback interface, lo0.

By default, the multicast-capable interface with the lowest-index address is chosen as the primary interface. If there is no such interface, the point-to-point interface with the lowest index address is chosen. Otherwise, any interface with an address could be picked. In practice, this means that, on the router, the fxp0 interface is picked by default.

To configure a different interface to be the primary interface, include the primary statement at the [edit interfaces interface-name unit logical-unit-number family family] hierarchy level:

[edit interfaces interface-name unit logical-unit-number family family] 

primary;