IS-IS IIH Circuit Type Values for Level-1-2 Routers
Short answer
B A Level-1-2 router sets the Circuit Type field in its IIH packets to 11 to indicate it participates in both IS-IS routing levels.
Question
Key Takeaways
The trap is confusing the binary codes for different router types; remember that 11 represents dual-level participation, whereas 01 and 10 represent single-level isolation.
In IS-IS networks, Level-1-2 routers send IIH packets with a Circuit Type value of 11. This distinguishes them from pure Level-1 (01) and Level-2 (10) routers during neighbor adjacency formation.
Common mistake: Choosing option C (01) or D (10). Learners often mix up the binary representations, thinking 01 might mean 'both' or forgetting that 11 is the specific code for the combined Level-1-2 role.
Expert Analysis
IS-IS uses a two-level hierarchy to organize routers into areas. The Circuit Type field in Intermediate System-to-Intermediate System Hellos (IIH) packets indicates the routing level of the sending router. This field is a single bit or part of the header that defines whether a router operates as Level-1, Level-2, or both.
The specific binary values assigned to these levels are:
- 01: Indicates a Level-1 router. These routers handle intra-area routing and send IIH packets with this type to discover other Level-1 neighbors.
- 10: Indicates a Level-2 router. These routers handle inter-area routing and use this value to discover Level-2 neighbors.
- 11: Indicates a Level-1-2 router. Since these routers participate in both levels, their IIH packets contain the value 11. This allows them to form adjacencies with both Level-1 and Level-2 routers on the same segment.
The question specifically asks for the value sent by a Level-1-2 router. Based on the IS-IS protocol specification, a Level-1-2 router must identify itself as such in its Hello packets to ensure proper neighbor discovery across area boundaries. Therefore, the Circuit Type field must be set to 11. Comments confirming this logic align with the standard protocol behavior where 01 is L1, 10 is L2, and 11 is L1/L2.
Exam Strategy
Memorize the binary mapping for IS-IS Circuit Types: 01 for Level-1, 10 for Level-2, and 11 for Level-1-2. When answering questions about IIH packet fields, always check if the router is multi-homing (L1-2) which requires the combined bit value to establish adjacencies on multiple levels simultaneously.
Frequently Asked Questions
What does the Circuit Type field in IS-IIH packets indicate?
Why do Level-1-2 routers use Circuit Type 11?
Community Votes
100% of anonymous learners picked answer B. Votes are pick records left by other test-takers — they are not the verified answer.
Discussion 1 contributions
Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.
From public community threads 1
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HCIP Resolutor 👍 1 Answered BEl valor 01 (binario) indica nivel Level-1. El valor 10 (binario) indica nivel Level-2. El valor 11 (binario) indica nivel Level-1-2.
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