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Exam 300-410 topic 1 question 58 discussion

Actual exam question from Cisco's 300-410

Question #: 58
Topic #: 1

After some changes in the routing policy, it is noticed that the router in AS 45123 is being used as a transit AS router for several service providers.
Which configuration ensures that the branch router in AS 45123 advertises only the local networks to all SP neighbors?
A.

B.

C.

D.

Show Suggested Answer

Suggested Answer: C

by Masashi_O at June 10, 2021, 12:08 p.m.

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Alexloh

Highly Voted 3 years ago

the regular expression (^$) matches any route that has an empty AS path attribute (that is, no character from start to end). Only locally originated routes have an empty AS path attribute; hence this regular expression is used when matching local routes. This type of filter is used by multihomed customers to send only their address space to their service providers, to prevent them from becoming a transit AS.

upvoted 18 times

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Mjestic

Highly Voted 3 years, 11 months ago

C is correct. -> https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/26634-bgp-toc.html#asregexp --> ^$ : This expression indicates origination from this AS.

upvoted 10 times

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conft

Most Recent 1 year, 11 months ago

C is correct.

upvoted 2 times

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Hack4

3 years, 5 months ago

C is correct

upvoted 1 times

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Dirkd0344

3 years, 8 months ago

It is C. The ^$ regex statement matches an empty AS Path. Only locally originated routes will have an empty AS Path.

upvoted 3 times

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[Removed]

3 years, 12 months ago

The correct answer is A ^$ ^ = begins with, $ = ends with. What BGP routes have no ASN!

upvoted 1 times

mosvan

3 years, 11 months ago

@examShark, Thank you for your contribution, but here you are wrong. Paths originating locally can be matched by ^$ and filter out to ISPs. So answer C is the correct one.

upvoted 3 times

vdsdrs

3 years, 11 months ago

While a newly sourced route is still within the AS in which it was created, the AS path is empty. When the AS has a requirement to filter out all but the routes that are local to itself before sending them to a neighboring AS, the AS will permit sending of the routes with the empty AS path and will deny all others. Answer C

upvoted 2 times

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RTE

4 years ago

It's B, locally orginitated - started with ASN of enterpise and ends on this ASN. Other uses match characterf at the end of matching string -$ Correct me

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Masashi_O

4 years, 1 month ago

C. Specify only the routes generated by your own AS.

upvoted 2 times

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