JN0-106 User Interfaces Practice Question
Exhibit
interfaces {
ge-0/0/0 {
unit 0 {
family inet {
address 10.0.0.1/24;
}
}
}
}Refer to the exhibit. What is the purpose of the 'unit 0' statement?
⚠ Common exam trap
The trap here is that candidates familiar with Cisco IOS might assume 'unit 0' is a physical interface or a default MTU setting, but in Junos, the unit number always defines a logical interface, and physical interfaces are configured separately without a unit keyword.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Defines a logical interface.
In Junos, the 'unit 0' statement is used to define a logical interface (also known as a subinterface) under a physical interface. Every physical interface must have at least one logical unit, and unit 0 is the default logical interface that carries Layer 3 configuration such as IP addresses. This is fundamental to Junos architecture, where all protocol configurations are applied at the logical unit level, not the physical interface level.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Enables IPv6.
Why it's wrong here
The unit statement in Junos does not enable IPv6; it defines a logical interface. IPv6 is activated by configuring the 'family inet6' statement under that logical interface (e.g., 'unit 0 { family inet6; }'). Without a unit, there is no place to attach the address family, but unit itself is only a container, not a protocol enabler.
- ✗
Sets the MTU.
Why it's wrong here
The unit statement does not set the MTU. MTU is configured separately using the 'mtu' statement, either at the physical interface level, under a logical unit, or per protocol family (e.g., 'unit 0 { family inet { mtu 1500; } }'). The unit simply identifies a distinct logical subinterface; MTU is a tunable parameter applied within that subinterface or on the physical interface.
- ✗
Defines a physical interface.
Why it's wrong here
A physical interface in Junos is defined by its interface name (e.g., 'ge-0/0/0'), not by the unit number. The 'unit' statement is always a child of a physical interface stanza and creates a logical (Layer 3 or Layer 2) subinterface. For example, 'ge-0/0/0 { unit 0 { ... } }'—the 'unit' is a subdivision of the physical interface, never the physical interface itself.
- ✓
Defines a logical interface.
Why this is correct
The 'unit 0' statement in Junos defines a logical interface on a physical interface. Logical interfaces allow you to configure separate Layer 3 properties (IP addresses, protocol families, VLAN tags) on the same physical port. Unit 0 is the default logical unit for untagged traffic, and additional units (1, 2, ...) correspond to subinterfaces or VLAN-tagged logical interfaces.
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