Discuss Cisco 300-410 Exam Topic 15 Question 98

Refer to the exhibit.

An engineer recently implemented uRPF by configuring the ip verify unicast source reachable-via rx command on interface gi0/3 The engineer noticed right after implementing F that an inbound packet on the giO-3 interface with a source address of 172 16 3 251 was dropped. Which action resolves the issue?

AConfigure uRPF loose mode to forward the packet.

BPermit the 172.16.3.251 in the inbound ACL on interface gi0/3.

CPermit the 172.16.3.251 in the inbound ACL on interface gi0/3 to allow 172.16.3.251.

DConfigure uRPF strict mode to forward the packet

Suggested Answer: A

Option A configures uRPF loose mode to forward the packet. This mode allows the router to check if there is a route in the routing table that matches the source IP address of the incoming packet, regardless of the interface that is used to reach the source.This mode is suitable for networks that have asymmetric routing, where the incoming and outgoing interfaces for a packet may differ2.

Option B permits the 172.16.3.251 in the inbound ACL on interface gi0/3. This option does not resolve the issue, because it only allows the packet to pass the ACL check, but not the uRPF check.The packet will still be dropped by uRPF if there is no route to reach the source IP address via the same interface3.

Option C permits the 172.16.3.251 in the inbound ACL on interface gi0/3 to allow 172.16.3.251. This option is redundant and incorrect, because it repeats the same IP address twice in the ACL statement.It also does not resolve the issue for the same reason as Option B3.

Option D configures uRPF strict mode to forward the packet. This option does not resolve the issue, because it is the same mode that was already configured on the interface. Strict mode requires that the router has a route to reach the source IP address via the same interface where the packet was received.If this condition is not met, the packet will be dropped4.

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28 comments · newest first

Lindy 11 months ago

Agree, ACL adjustment seems like the best fix.

Darrin 11 months ago

Surprised it dropped that packet! Thought it would be allowed.

Casandra 11 months ago

I think loose mode could work here.

Cassi 11 months ago

Definitely need to check the ACL for that IP.

Johnson 12 months ago

uRPF strict mode drops packets not seen in the routing table.

Susana 12 months ago

I vaguely recall that loose mode allows more flexibility, but I’m not confident if that’s the best option for this scenario.

Quentin 1 year ago

I practiced a similar question, and I think uRPF strict mode would actually drop the packet too, so that's probably not the answer.

Isabelle 1 year ago

I'm not entirely sure, but I feel like permitting the IP in the ACL could resolve the issue. It seems like a common practice question.

Jennie 1 year ago

I remember studying uRPF and how it can drop packets if the source isn't reachable. I think loose mode might help here.

Tenesha 1 year ago

I'm pretty confident that the right answer is to permit the 172.16.3.251 address in the inbound ACL. That should resolve the issue with the uRPF drop.

Mitzie 1 year ago

Okay, the key here is that the packet was dropped due to the uRPF configuration. So I think the solution is to either configure uRPF loose mode or add the source address to the ACL.

Dominic 1 year ago

Hmm, I'm a bit confused about the difference between uRPF loose and strict modes. I'll have to review that before deciding on the best answer.

Anna 1 year ago

This looks like a tricky uRPF question. I'll need to carefully read through the options and think about how uRPF works.

Shawna 1 year ago

I'm leaning towards option D - configuring uRPF strict mode. That seems like it would be the most secure approach to resolving the problem.

Brandon 2 years ago

Hmm, that makes sense too. Let's see what others think before the exam.

Keena 2 years ago

I disagree, I believe the correct answer is D) Configure uRPF strict mode to forward the packet.

Theodora 2 years ago

Option A is the way to go, my friend. Loose mode will let the packet through, no problem.

Augustine 2 years ago

↳ replying to Theodora

D) Configure uRPF strict mode to forward the packet

Junita 2 years ago

↳ replying to Theodora

Got it, thanks for the advice!

Devorah 2 years ago

↳ replying to Theodora

B) Permit the 172.16.3.251 in the inbound ACL on interface gi0/3.

Theresia 2 years ago

↳ replying to Theodora

A) Configure uRPF loose mode to forward the packet.

Mohammad 2 years ago

Haha, I'm going to have to go with Option C. Gotta love those redundant answers that make you scratch your head!

Dylan 2 years ago

I think Option B is the way to go. Permitting the source address in the ACL on the interface will allow the packet to pass through.

Brandon 2 years ago

I think the answer is B) Permit the 172.16.3.251 in the inbound ACL on interface gi0/3.

Elmira 2 years ago

Option D makes the most sense. If the issue is that a packet with a source address of 172.16.3.251 is getting dropped, then we need to configure uRPF strict mode to forward the packet.

Jacklyn 2 years ago

↳ replying to Elmira

That should resolve the issue with the dropped packet.

Viva 2 years ago

↳ replying to Elmira

We should configure uRPF strict mode to forward the packet.

Vernell 2 years ago

↳ replying to Elmira

Option D makes the most sense.

Cisco 300-410 Exam - Topic 15 Question 98 Discussion