Discuss Ericsson ECP-206 Exam Topic 2 Question 28

What does an LDP implicit null label cause?

Aswapping with a new label

Bpenultimate hop popping by a downstream router

Cforwarding the packet unchanged

Dpenultimate hop popping by an upstream router

Suggested Answer: B

An LDP implicit null label causes penultimate hop popping by a downstream router. LDP is a protocol that distributes labels for MPLS forwarding along the shortest path calculated by an IGP. An implicit null label is a special label value of 3 that indicates that the downstream router does not need a label to forward the packet to its destination. When an upstream router receives an implicit null label from a downstream router, it removes (or pops) the label from the packet before sending it to the downstream router.This process is called penultimate hop popping (PHP) and it reduces the load on the downstream router, which can forward the packet based on its IP header or another label in the stack59.

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35 comments · newest first

Franklyn 10 months ago

Nope, definitely not that! It's about swapping labels.

Catarina 11 months ago

I thought it was about forwarding the packet unchanged.

Chauncey 11 months ago

Wait, is it really just that? Seems too simple.

Ciara 11 months ago

Totally agree with that!

Theresia 11 months ago

It causes penultimate hop popping by a downstream router.

Goldie 12 months ago

I vaguely recall that it has to do with the penultimate hop popping, but I can't remember if it's upstream or downstream.

Hana 12 months ago

I feel like the answer might be about forwarding the packet unchanged, but that doesn't seem right for implicit null.

Emily 12 months ago

I remember practicing a similar question, and I think it was about how the implicit null label affects packet forwarding.

Dierdre 1 year ago

I think the implicit null label is related to penultimate hop popping, but I'm not sure if it's upstream or downstream.

Fernanda 1 year ago

I'm a bit confused by this question. I know LDP is related to MPLS, but I'm not sure about the specifics of implicit null labels and how they affect packet forwarding. I'll have to do some quick review of MPLS concepts before answering this.

Leota 1 year ago

Okay, I've got this. An LDP implicit null label causes penultimate hop popping by a downstream router. I remember learning about this in class, where the downstream router removes the label and forwards the packet based on the IP destination address. I'm confident that's the right answer.

Cathern 1 year ago

I'm not too sure about this one. The question seems to be asking about LDP implicit null labels, but I'm not familiar with the specifics of how those work. I'll have to think this through carefully.

Coral 1 year ago

Hmm, this is a tricky one. I think it has to do with how the label is handled by downstream routers, but I'm not sure if it's popping the label or forwarding it unchanged. I'll have to review my notes on LDP to figure this out.

Jordan 2 years ago

I'm just going to go with B. Penultimate hop popping by a downstream router sounds like the most sensible option to me.

Gail 2 years ago

Wait, is there a trick question here? I thought LDP implicit null labels were supposed to make things easier, not more confusing!

Barbra 2 years ago

↳ replying to Gail

C) forwarding the packet unchanged

Staci 2 years ago

↳ replying to Gail

B) penultimate hop popping by a downstream router

Annett 2 years ago

↳ replying to Gail

A) swapping with a new label

Vashti 2 years ago

I'm not sure, but I think it might be D) penultimate hop popping by an upstream router instead.

Daron 2 years ago

I agree with Alyce, because the implicit null label is used to signal the penultimate hop to pop the label.

Celestina 2 years ago

D has to be the answer. Penultimate hop popping by an upstream router is the expected behavior, right?

Katheryn 2 years ago

↳ replying to Celestina

That's correct, the LDP implicit null label causes penultimate hop popping by an upstream router.

Goldie 2 years ago

↳ replying to Celestina

I agree, it causes penultimate hop popping by an upstream router.

Virgie 2 years ago

↳ replying to Celestina

Yes, penultimate hop popping by an upstream router is the expected behavior.

Theola 2 years ago

↳ replying to Celestina

I think you're right, D is the answer.

Maryln 2 years ago

I think C is the correct answer. Forwarding the packet unchanged seems like the logical choice for an LDP implicit null label.

Essie 2 years ago

↳ replying to Maryln

Yes, that's correct. The LDP implicit null label causes the packet to be forwarded unchanged.

Javier 2 years ago

↳ replying to Maryln

I agree, forwarding the packet unchanged makes sense.

Teddy 2 years ago

↳ replying to Maryln

I think C is the correct answer.

Shaun 2 years ago

Option B makes sense to me. The penultimate hop popping by a downstream router seems like the right behavior for an LDP implicit null label.

Mindy 2 years ago

↳ replying to Shaun

No, it actually causes penultimate hop popping by a downstream router.

Dick 2 years ago

↳ replying to Shaun

So, the packet is forwarded unchanged?

Hildred 2 years ago

↳ replying to Shaun

Yes, that's correct. It is the behavior for an LDP implicit null label.

Tayna 2 years ago

↳ replying to Shaun

I think it causes penultimate hop popping by a downstream router.

Alyce 2 years ago

I think the answer is B) penultimate hop popping by a downstream router.

Ericsson ECP-206 Exam - Topic 2 Question 28 Discussion