Option A (Correct):
Complete Sequence Number PDUs (CSNPs) are periodically flooded by the Designated Intermediate System (DIS) on multi-access networks (e.g., Ethernet).
This ensures all routers on the segment synchronize their Link-State Databases (LSDBs).
Reference:
Option C (Correct):
Partial Sequence Number PDUs (PSNPs) contain only the headers (descriptions) of LSPs (e.g., LSP ID, sequence number, checksum).
PSNPs are used to:
Request missing LSPs (when a router detects discrepancies via CSNPs).
Acknowledge LSP receipt (in point-to-point networks).
They do not include the full LSP data.
Why Other Options Are Incorrect:
Option B: Incorrect. PSNPs are not flooded periodically-they are sent on-demand for specific LSP synchronization.
Option D: Incorrect. While CSNPs do contain LSP descriptions (headers), the term "only" is misleading. CSNPs summarize all LSPs in the LSDB, but they are not limited to "only" descriptions-they serve as a complete database overview.
Key Takeaways:
CSNPs are periodic, broadcast by the DIS, and ensure LSDB consistency.
PSNPs are triggered, contain specific LSP headers, and handle requests/acknowledgments.
IS-IS uses CSNPs and PSNPs to maintain efficient LSDB synchronization without flooding full LSPs unnecessarily.
For further details, refer to Juniper's official IS-IS documentation:
Juniper IS-IS Configuration Guide.