What are the main components of the SNAP address used by the IS-IS protocol? (Multiple choice)

Question

Options

• AOrigin
• BSEL
• COSCP
• DSystem ID

• A
  83% (25)
• B
  10% (3)
• C
  3% (1)
• D
  3% (1)

Explanation

Important note before the explanation: The provided correct answer (A: Origin) appears to conflict with standard IS-IS networking knowledge. This may be an error in the exam key.

What IS-IS NSAP/SNAP addresses actually contain:

IS-IS uses NSAP (Network Service Access Point) addresses - sometimes called SNAP addresses - with three core components:

1. System ID (Choice D) - a 6-byte identifier, typically derived from a MAC address, that uniquely identifies a router within an area. This is unambiguously a real component.

2. SEL/NSEL (Choice B) - the N-Selector (1 byte), which identifies the network layer entity. IS-IS routers always set this to 0x00. Also a genuine component.

3. Area Address - the remaining prefix that identifies the routing area. Neither "Origin" nor any of the other choices maps cleanly to this.

Why each choice is problematic:

• A (Origin) - "Origin" is not a standard IS-IS NSAP field name. It may be a distractor borrowed from BGP (which has an "Origin" attribute).
• C (OSCP) - not a real IS-IS term; likely a confuser blending OSPF and IS-IS terminology.

Memory tip: Think "Area + SID + SEL = NSAP" - like a postal address: area code, local ID, apartment selector. If your exam uses the term "Origin," map it mentally to the Area Address portion.

Recommendation: Verify this question against your course material, as choices B and D are the textbook-correct NSAP components.

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