350-501 Networking Practice Question
What is the purpose of the SEL (NSAP Selector) field in an IS-IS NET address?
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
✓
It is always set to 0x00 for the device itself.
In IS-IS, the NET (Network Entity Title) address includes an SEL byte. The SEL is always set to 0x00 for the device itself, as it identifies the network layer entity, not a specific service.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
It identifies the system ID of the router.
Why it's wrong here
System ID is a separate field in the NET address.
- ✓
It is always set to 0x00 for the device itself.
Why this is correct
Correct. SEL is 0x00 for the network layer entity.
- ✗
It is used to select the routing protocol.
Why it's wrong here
SEL does not select the routing protocol.
- ✗
It identifies the area within the routing domain.
Why it's wrong here
Area identification is done by the Area ID portion, not SEL.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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