IS-IS Loopback Route Advertisement Analysis
Short answer
BD R1's routing table contains 10.0.2.2/32 but lacks 10.0.2.3/32 due to IS-IS being disabled on R2's Loopback3.
Question
Key Takeaways
The core trap is assuming all loopbacks are advertised; the key insight is that IS-IS only advertises prefixes from interfaces where the process is explicitly enabled.
This H12-821 question examines how IS-IS interface configuration affects route propagation, specifically why disabling IS-IS on a loopback prevents its route from appearing in neighbor routing tables.
Common mistake: Candidates often select Option A, incorrectly assuming that all loopback interfaces are automatically advertised into the IS-IS domain regardless of interface-level protocol enablement.
Expert Analysis
The question tests IS-IS Level-1/Level-2 adjacency behavior and route advertisement regarding loopback interfaces. R1 and R2 establish an IS-IS adjacency, which implies they are in the same area (typically Level-1) or adjacent levels. The key configuration on R2 is that IS-IS is enabled on Loopback0 but disabled on Loopback3. In IS-IS, only interfaces with the protocol enabled advertise their connected routes into the IS-IS database via LSPs (Link State PDUs). Therefore, R2 will originate an LSP containing the prefix of Loopback0 (10.0.2.2/32) but will not originate an LSP for Loopback3 (10.0.2.3/32). Consequently, R1 receives the LSP for 10.0.2.2/32 and installs it in its routing table. It does not receive any LSP for 10.0.2.3/32, so that route is absent. Option A is incorrect because the route is not advertised. Option C is incorrect because the 10.0.2.2/32 route is present. Option B is correct as the specific route for Loopback3 is missing. Option D is correct as the route for Loopback0 is present.
Exam Strategy
When analyzing IS-IS questions, always check the specific interface configuration first. Remember that IS-IS operates at Layer 2 (Data Link), but IP prefixes are only injected into the topology if the interface is explicitly activated under the IS-IS process. If an interface is down or IS-IS is disabled on it, no LSP is generated for that prefix.
Frequently Asked Questions
Does IS-IS automatically advertise all loopback interfaces?
What happens if IS-IS is disabled on a physical interface?
Community Votes
50% of anonymous learners picked answer A. 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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Adi_1 👍 1A and D are True
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