IS-IS Backbone Router Types
Short answer
B Level-1-2 and Level-2 routers can exist in the IS-IS backbone area.
Question
Key Takeaways
The trap is confusing IS-IS backbone with OSPF Area 0; remember that any router participating in L2 routing is part of the IS-IS backbone.
This question tests knowledge of IS-IS hierarchy, specifically identifying that both Level-2 and Level-1-2 routers reside in the backbone.
Common mistake: Choosing C (Level-2 only) ignores that Level-1-2 border routers also connect to the backbone via L2 adjacencies.
Expert Analysis
In IS-IS, the backbone is defined as the collection of all Level-2 (L2) capable routers and the contiguous L2 link layer that connects them. Unlike OSPF where Area 0 is a specific logical area, IS-IS backbone is functional rather than geographical.
To determine which routers exist in the backbone, we must look at their capabilities:
- Level-2 Routers: These are exclusively responsible for inter-area routing and form the core of the backbone. They are always present in the backbone.
- Level-1-2 Routers: These routers act as border routers between an area and the backbone. To route traffic between its local Level-1 area and the rest of the network, it must participate in the Level-2 domain. Therefore, it maintains L2 adjacencies and is considered part of the backbone structure.
- Level-1 Routers: These operate solely within their own area and do not maintain L2 adjacencies. They are strictly non-backbone.
The Spanish comment confirms this: "Level-2: Pertenecen exclusivamente al backbone... Level-1-2:...forman parte del backbone Level-2". Since both L2 and L1-2 routers participate in the Level-2 backbone, Option B is the correct classification.
Exam Strategy
When comparing IS-IS to OSPF, map concepts carefully: IS-IS 'Backbone' (L2 domain) ≈ OSPF 'Area 0'. Remember that border routers in IS-IS (L1-2) are part of the backbone because they run L2, unlike OSPF ABRs which are technically in Area 0 but often treated distinctly.
Frequently Asked Questions
What is the difference between IS-IS backbone and OSPF Area 0?
Can a Level-1 router be in the backbone?
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 BLevel-2: Pertenecen exclusivamente al backbone. Se encargan del enrutamiento inter-área. Level-1-2: Son routers frontera de área (equivalentes a los ABRs de OSPF). Tienen adyacencias tanto Level-1 como Level-2 y forman parte del backbone Level-2.
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