Confused by BGP solution from question 2

Confused by BGP solution from question 2

by Tiannan Sha -
Number of replies: 3

Hi,

I was confused during the exam and still am about everything starting from question 2. What are the best route for routers in B? I would argue even with the local pref added they would still be via routers in A, for lower identifier compared to AS path BCD. Why would they advertise BCD to A??

Best regards,

Tiannan


In reply to Tiannan Sha

Re: Confused by BGP solution from question 2

by Plouton Grammatikos -

Hello, 

You are right that the best route from AS B will be through AS A. However, if a router in B receives a packet from A, it will not forward it back to A, because that would create a loop. The way routers avoid the loops is by putting only a subset of the routes from RIB-in to RIB-out. Please check the example in slide 28 of bgpsols.

Best,

Plouton

In reply to Plouton Grammatikos

Re: Confused by BGP solution from question 2

by Tiannan Sha -

But why wouldn’t it work like this:

B’s best route is through A. And because A cannot forward back to B to form loop, A’s best route is still directly to D?

If B’s best route is via A, why would it export BCD to A?


In reply to Tiannan Sha

Re: Confused by BGP solution from question 2

by Plouton Grammatikos -
Dear Tiannan,

It turns out you are right. In fact, there are two possible solutions that depend on the order the BGP announcements are sent by the routers. We have updated the exam solutions on moodle to include this alternative solution. Thank you very much for your observation!

Whoever had this alternative solution in the exam, that was incorrectly marked as wrong, will be notified about their updated grade shortly.

We apologize for our missunderstanding of the problem.

Best,
Plouton