Intermediate System to Intermediate System (IS-IS) is a link-state routing protocol used to move information efficiently within a computer network. It uses a series of Protocol Data Units (PDUs) to manage the network's topology and ensure consistency across all routers in the network. Specifically, Link State PDUs (LSPs), Complete Sequence Number PDUs (CSNPs), and Partial Sequence Number PDUs (PSNPs) play crucial roles in this process.
1. **PSNPs (Partial Sequence Number PDUs)**:
- **Acknowledge a received LSP**: PSNPs are used to acknowledge the receipt of LSPs. When a router receives an LSP, it sends a PSNP back to the sender to confirm that the LSP has been received.
- **Request a missing LSP**: PSNPs are also used to request missing LSPs. If a router identifies a missing LSP based on sequence numbers, it can send a PSNP to request the specific LSP from its neighbors.
2. **CSNPs (Complete Sequence Number PDUs)**:
- **Summarize LSPs**: CSNPs are used to summarize all the LSPs known to a router. They are typically sent at regular intervals to provide a complete list of LSPs in a database. They are not used to acknowledge or request specific LSPs but provide an overview of all LSPs for database synchronization.
Based on this understanding, let's evaluate the statements:
- **A. PSNPs are used to acknowledge a received LSP.**
- Correct. PSNPs serve the purpose of acknowledging LSPs received from other routers.
- **B. CSNPs are used to acknowledge a received LSP.**
- Incorrect. CSNPs are not used for acknowledging LSPs; they are used to provide a summary of all LSPs.
- **C. CSNPs are used to request a missing LSP.**
- Incorrect. CSNPs are not used to request missing LSPs; this is the role of PSNPs.
- **D. PSNPs are used to request a missing LSP.**
- Correct. PSNPs are used to request specific missing LSPs when a router detects that it is missing information.
**Conclusion**:
The correct statements about IS-IS are:
**A. PSNPs are used to acknowledge a received LSP.**
**D. PSNPs are used to request a missing LSP.**
**References**:
- Juniper Networks Documentation on IS-IS: [IS-IS Overview](https://www.juniper.net/documentation/en_US/junos/topics/concept/is-is-routing-overview.html)
- RFC 1195, Use of OSI IS-IS for Routing in TCP/IP and Dual Environments: [RFC 1195](https://tools.ietf.org/html/rfc1195) which details the operation and use of IS-IS, including the roles of PSNPs and CSNPs.
