Which two statements are true about the primary address on an interface? (Choose two.)
A.
It is the address used by default as the local address for broadcast and multicast packets
sourced locally and sent out of the interface.
B.
You use the primary address when you have multiple IP addresses belonging to the same
subnet on the same interface.
C.
It can be useful for selecting the local address used for packets sent out of unnumbered
interfaces when multiple non-127 addresses are configured on the loopback interface.
D.
By default, the primary address on an interface is selected as the numerically highest local
address configured on the interface.
Explanation:
Topic 2, Volume B
7.10. Choosing Primary and Preferred Interface Addresses
Problem
You have an Ethernet or other point-to-multipoint interface and have to set up multiple addresses on the interface—for example, so you can treat class of service for each flow differently—but you need to always use one of the addresses first.
Solution
Use the following commands to pick one address to always be the source address for traffic on the same subnet:
[edit interfaces fe-0/0/0]
aviva@router1# set unit 0 family inet address 192.168.220.1/24
aviva@router1# set unit 0 family inet address 192.168.220.2/24
preferred
Use the following commands to choose one address that is used as the source address in broadcast and unnumbered traffic sent out an interface:
[edit interfaces fe-0/0/1]
aviva@router1# set unit 0 family inet address 192.168.220.1/24
aviva@router1# set unit 0 family inet address 192.168.220.2/24
primary
Discussion
Each JUNOS interface has a preferred address, which is the default local address used when there is more than one address in the same subnet on the same interface. Each JUNOS interface also has a primary address, which is used by default as the source address when you originate packets out the interface where the destination gives no hint about the subnet. By default, the software chooses the numerically lowest address as the preferred and primary address. The show interfaces command output indicates which addresses are the preferred and primary:
aviva@router1> show interfaces fe-0/0/0.0 Logical
https://www.safaribooksonline.com/library/view/junos-cookbook/0596100140/ch07s10.html
http://www.juniper.net/documentation/en_US/junos11.4/topics/reference/configuration-statement/default-address-selection-edit-system-qfx-series.html
default-address-selection
Syntax
default-address-selection;
Hierarchy Level
[edit system]
Release Information
Statement introduced in Junos OS Release 11.1 for the QFX Series.
Description
Use the loopback interface, lo0, as the source address for all locally generated IP packets when the packet is sent through a routed interface, but not when the packet is sent through a local interface such as fxp0. The lo0 interface is the interface to the switch’s Routing Engine.
Default
The default address is used as the source address for all locally generated IP packets on outgoing interfaces that are unnumbered. If an outgoing interface is numbered, the default address is chosen using the following sequence:
The primary address on the loopback interface lo0 that is not 127.0.0.1 is used.
The primary address for the primary interface or the preferred address (if configured) for the primary interface is used.
By default, the primary address on an interface is selected as the numerically lowest local address configured on the interface.
An interface’s primary address is used by default as the local address for broadcast and multicast packets sourced locally and sent out through the interface. An interface’s preferred address is the default local address used for packets sourced by the local switch to destinations on the subnet. By default, the numerically lowest local address configured for the interface is chosen as the preferred address on the subnet.
To configure a different primary address or preferred address, include the primary or preferred statement at the [edit interfaces interface-name unit logical-unit-number family family address address] or [edit logical-systems logical-system-name interfaces interface-name unit logical-unit-number family family address address] hierarchy levels.
Required Privilege Level
system—To view this statement in the configuration.
system-control—To add this statement to the configuration.
http://www.juniper.net/documentation/en_US/junos14.2/topics/reference/configuration-statement/primary-edit-interfaces.html