Discuss Juniper JN0-664 Exam Topic 5 Question 33

Which two statements about IS-IS are correct? (Choose two.)

APSNPs are used to acknowledge a received LSP.

BCSNPs are used to acknowledge a received LSP.

CCSNPs are used to request a missing LSP.

DPSNPs are used to request a missing LSP.

Suggested Answer: A, D

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.**

**Reference**:

- 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.

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31 comments · newest first

Vanesa 11 months ago

CSNPs are for acknowledging, no doubt about it!

Kristeen 11 months ago

Wait, are you sure about that? I thought it was the other way around.

Jolene 11 months ago

Totally agree, PSNPs are used for requests!

Jeanice 11 months ago

I thought CSNPs were for acknowledgments?

Han 12 months ago

PSNPs are definitely for requesting missing LSPs.

Barrett 12 months ago

I’m a bit confused about PSNPs and CSNPs; I thought both had to do with LSP acknowledgment, but now I’m not so sure.

Miesha 1 year ago

I practiced a question similar to this, and I feel like CSNPs are definitely for requesting missing LSPs.

Tamesha 1 year ago

I remember CSNPs being involved in acknowledging LSPs, but I might be mixing it up with something else.

Rolande 1 year ago

I think PSNPs are related to acknowledging LSPs, but I'm not entirely sure if that's correct.

Staci 1 year ago

I'm confident I can get this one right. The key is remembering the differences between PSNPs and CSNPs in an IS-IS network.

Johana 1 year ago

Okay, I know PSNPs and CSNPs are related to Link State Packets (LSPs), but I'm not totally sure about the specifics. I'll have to think this through.

Annelle 1 year ago

Hmm, this is a tricky one. I'll need to review my notes on IS-IS packet types to make sure I get this right.

Jani 1 year ago

I'm pretty familiar with IS-IS, so I think I can handle this one. Let me think through the options carefully.

Yuriko 2 years ago

Wait, what's an LSP again? I'm going to play it safe and choose A and B, just to be safe.

Reena 2 years ago

Haha, this question is a real brain-teaser! I'm going to go with B and C, just to keep everyone on their toes.

Alesia 2 years ago

↳ replying to Reena

Great choices! Those are the two correct statements about IS-IS.

Vicente 2 years ago

↳ replying to Reena

I agree, and C is also correct because PSNPs contain only descriptions of LSPs.

Britt 2 years ago

↳ replying to Reena

I think B is correct because PSNPs are flooded periodically.

Linsey 2 years ago

This is a tricky one, but I'm going to guess A and D. It just makes sense that CSNPs would contain the descriptions of LSPs, right?

Bea 2 years ago

Hmm, I think C and D are the correct answers. CSNPs and PSNPs contain descriptions of LSPs, not just the LSPs themselves.

Clarinda 2 years ago

↳ replying to Bea

Yes, C and D are the correct answers. CSNPs and PSNPs contain descriptions of LSPs.

Telma 2 years ago

↳ replying to Bea

CSNPs and PSNPs do contain descriptions of LSPs, not just the LSPs themselves.

Corrinne 2 years ago

↳ replying to Bea

That's correct, CSNPs and PSNPs are flooded periodically.

Sommer 2 years ago

↳ replying to Bea

I agree, CSNPs and PSNPs both contain descriptions of LSPs.

Agustin 2 years ago

I agree with Arthur, PSNPs containing only descriptions of LSPs sounds right to me.

Justine 2 years ago

I'm pretty sure A and B are the correct answers. CSNPs and PSNPs are both flooded periodically to keep the network topology up-to-date.

Brynn 2 years ago

↳ replying to Justine

Yes, that's correct. It helps to keep the network topology current.

Franklyn 2 years ago

↳ replying to Justine

I think you're right, CSNPs and PSNPs are both flooded periodically.

Arthur 2 years ago

I disagree, I believe B) and D) are the correct statements.

Pete 2 years ago

I'm not sure, but I think A) makes sense because CSNPs are indeed flooded periodically.

Lyda 2 years ago

I think A) and C) are correct.

Juniper JN0-664 Exam - Topic 5 Question 33 Discussion