Problem 2.3 from Past Paper 2019

Problem 2.3 from Past Paper 2019

by Xinran Tao -
Number of replies: 1

Dear all,

I am confused by the answer to the following question:

Given the following network configuration:


And the following BGP decision process:

  1. Shortest AS-PATH 
  2. E-BGP is preferred over I-BGP 
  3. Shortest path to NEXT-HOP, according to IGP 
  4. Lowest BGP identifier is preferred (e.g. A1 is preferred over A2)

B2 received two routes to 4001:1::/32, one from B3 (NEXT-HOP=c2e) and one from B4 (NEXT-HOP=c1e). Part of the given answer is "...the other one, obtained from B4 is ranked equally but has a larger BGP identifier..". If I understand correctly, the cost to go from B2 to B4 is always higher than that of B2 to B3. In this case, shouldn't B2 choose the route via B3 because it is the shortest path to NEXT-HOP according to IGP (i.e. OSPF that runs Dijkstra)?

Thank you very much for your time to answer my question.

Kind regards,

Paulina

In reply to Xinran Tao

Re: Problem 2.3 from Past Paper 2019

by Emi Sakamoto -
Dear Paulina,

You're right, the packets forwarded to B2 will then be forwarded to B3 because of the shortest path, not the BGP identifier. So there is an error in the provided answer.

Kind regards,
Emi