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AS number 144 has two border routers BR1 and BR2. BR1 peers with AS 555. BR2 peers with AS 1977. Assuming that BR2 decides to advertise a route to the prefix 128.15.0.0/16, which of the following are valid route advertisements? AS 144 may have received advertisements from other ASes but received no other advertisements for that prefix from AS 555. Check all that are valid.

1) 128.15.0.0/16 6294 144
2) 128.15.0.0/16 555 144
3) 128.15.0.0/16 78 11101 6294 555
4) 128.15.0.0/16 78 11101 6294 555 144
5) 128.15.0.0/16 78 11101 6294 555
6) 128.15.0.0/16 78 11101 6294 555 555 144 144
7) 128.15.0.0/16 78 78 144 144
8) 128.15.0.0/16 64393 144 144

I know that 3 equal to 5. Also previous question contain this:
BR1 receives a route advertisement from AS 555 for prefix 128.15.0.0/16, with path 78 11101 6294 555 555. The double listing of 555 is correct; it’s a way for 555 to make the path seem “longer.”

I've searched for AS_PATH examples but still don't understand this question: if I understand correctly network 128.15.0.0/16 in AS 555 as path is ended with 555. But why doesn't it have 144 in its beginning.

HaveNoDisplayName
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Dmitry Senkovich
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  • If there's no additional information the question looks wrong, can you post the full question? – Vasilis Aug 17 '15 at 21:29
  • No, this is full question. Though question is out of date, thanks! – Dmitry Senkovich Aug 20 '15 at 14:50
  • I believe the answer is 1, 7, and 8. 2-6 are invalid because of the 555 hop. The question doesn't say whether or not the router could be prepending, so based upon that assumption 7 and 8 are validate advertisements. – Wedge Martin Dec 16 '15 at 00:27

1 Answers1

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After many tries, I finally got the right answer, which is (1, 6, 7, 8). I am going to understand the answer in the following way. Because the question asks the AS path from somewhere to BR2, which peers with AS 1977, so the correct answer should end with AS 144 or AS 1977, because we only know these two are connected to BR2. Therefore, we must choose from (1, 2, 4, 6, 7, 8). Then, since the question states that the only advertisement from AS 555 with that prefix is (78 11101 6294 555 555), so there must be two 555 in the path. Therefore, (2, 4) are not right. (1, 7, 8) are possible because they do not pass AS 555, so there are possibilities they directly connected to AS 144.

I do not know the right explanation, I just try to give a reasonable explanation to the right answer. Maybe the whole question is wrong.

zangsy
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  • That’s a question from their tests in online networking course at Stanford. I was taking the course several years ago when I was a student so I can’t clarify more on the question. It was copy-and-pasted originally, I believe, so I wouldn’t say it is wrong or so – Dmitry Senkovich Jun 29 '19 at 20:57
  • No problem. I just want to let those people who will take the course in the future can find a proper answer when they tried to google it. – zangsy Jun 30 '19 at 21:09