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Exam 350-401 topic 1 question 168 discussion

Actual exam question from Cisco's 350-401

Question #: 168
Topic #: 1

Refer to the exhibit. An engineer must ensure that all traffic entering AS 200 from AS 100 chooses Link 2 as an entry point. Assume that all BGP neighbor relationships have been formed and that the attributes have not been changed on any of the routers. Which configuration accomplishes this task?

A. R3(config)#route-map PREPEND permit 10 R3(config-route-map)#set as-path prepend 200 200 200 R3(config)#router bgp 200 R3#(config-router)#neighbor 10.1.1.1 route-map PREPEND out
B. R4(config)#route-map PREPEND permit 10 R4(config-route-map)#set as-path prepend 100 100 100 R4(config)#router bgp 200 R4(config-router)#neighbor 10.2.2.2 route-map PREPEND in
C. R4(config)#route-map PREPEND permit 10 R4(config-route-map)#set as-path prepend 200 200 200 R4(config)#router bgp 200 R4(config-router)#neighbor 10.2.2.2 route-map PREPEND out
D. R3(config)#route-map PREPEND permit 10 R3(config-route-map)#set as-path prepend 100 100 100 R3(config)#router bgp 200 R3(config-router)#neighbor 10.1.1.1 route-map PREPEND in

Show Suggested Answer

Suggested Answer: A 🗳️

by [deleted] at May 10, 2021, 1:50 p.m.

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xziomal9

Highly Voted 3 years, 3 months ago

A. R3(config)#route-map PREPEND permit 10 R3(config-route-map)#set as-path prepend 200 200 200 R3(config)#router bgp 200 R3#(config-router)#neighbor 10.1.1.1 route-map PREPEND out B. R4(config)#route-map PREPEND permit 10 R4(config-route-map)#set as-path prepend 100 100 100 R4(config)#router bgp 200 R4(config-router)#neighbor 10.2.2.2 route-map PREPEND in C. R4(config)#route-map PREPEND permit 10 R4(config-route-map)#set as-path prepend 200 200 200 R4(config)#router bgp 200 R4(config-router)#neighbor 10.2.2.2 route-map PREPEND out D. R3(config)#route-map PREPEND permit 10 R3(config-route-map)#set as-path prepend 100 100 100 R3(config)#router bgp 200 R3(config-router)#neighbor 10.1.1.1 route-map PREPEND in

upvoted 32 times

XalaGyan

3 years ago

thx a lot bro for taking the time and writing this down. your answer is correct.

upvoted 3 times

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brightsyds

Highly Voted 2 years, 10 months ago

A for sure! R3 prepends its AS and sends it outbound to R1 in AS 100. Because R1 and R2 are in one iBGP domain (AS 100), all traffic from AS 100 will prefer to use a shorter AS path to AS 200 which is R4's link.

upvoted 12 times

timtgh

2 years, 7 months ago

Thanks. But why is it prepending it three times?

upvoted 4 times

bora4motion

2 years, 1 month ago

That's what you do when you want to "make a route look bad" you add your own AS to the AS-Path and make it longer. BGP is routing through AS numbers.

upvoted 8 times

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alawi2

2 years, 7 months ago

to make longer, imagine 200 + 200 + 200 it is basically telling BGP the equivalent of having to cross 3 AS with cost 200 each

upvoted 13 times

jason2626

2 years, 5 months ago

Makes sense! Thanks for the explanation.

upvoted 4 times

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[Removed]

Most Recent 7 months, 1 week ago

Selected Answer: A

A is correct

upvoted 1 times

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danman32

1 year, 5 months ago

I got this one wrong and selected D because I remembered similar Q139 and misread this question that we want AS100 to choose link 2, and not AS 200 choose link 2. But now that I read these discussions, realized I had the traffic direction backwards, in which case answer is definitely A.

upvoted 1 times

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charafDZ

1 year, 10 months ago

What is AS path Prepending? In the Border Gateway Protocol (BGP), prepending is a technique used to deprioritize a route by artificially increasing the length of the AS-PATH attribute by repeating an autonomous system number (ASN). Route selection in BGP prefers the shorter AS path length, assuming all other criteria are equal

upvoted 2 times

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Dataset

1 year, 10 months ago

hi! why is the route map PREPEND applied on R3 , out ? Thanks Regards

upvoted 1 times

danman32

1 year, 5 months ago

Because you want to influence AS100's traffic pattern to AS200, unlike Q139 which had you influence traffic from AS200 to AS100. Therefore you want link1 to appear to AS200 as the longer route.

upvoted 2 times

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bora4motion

2 years, 1 month ago

Selected Answer: A

A is the correct option, you add your own AS in the AS Path to make it "look bad".

upvoted 1 times

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WINDSON

2 years, 1 month ago

what is the meaning of permit 10 ?

upvoted 1 times

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Pudu_vlad

2 years, 3 months ago

A is correct

upvoted 1 times

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Raju255

2 years, 9 months ago

Correct Answer is A A. R3(config)#route-map PREPEND permit 10 R3(config-route-map)#set as-path prepend 200 200 200 R3(config)#router bgp 200 R3#(config-router)#neighbor 10.1.1.1 route-map PREPEND out B. R4(config)#route-map PREPEND permit 10 R4(config-route-map)#set as-path prepend 100 100 100 R4(config)#router bgp 200 R4(config-router)#neighbor 10.2.2.2 route-map PREPEND in C. R4(config)#route-map PREPEND permit 10 R4(config-route-map)#set as-path prepend 200 200 200 R4(config)#router bgp 200 R4(config-router)#neighbor 10.2.2.2 route-map PREPEND out D. R3(config)#route-map PREPEND permit 10 R3(config-route-map)#set as-path prepend 100 100 100 R3(config)#router bgp 200 R3(config-router)#neighbor 10.1.1.1 route-map PREPEND in

upvoted 3 times

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