Explore the optimal BGP design for managing traffic flow between autonomous systems, ensuring resilient connectivity to critical servers in a complex network topology.
Table of Contents
Question
Refer to the exhibit.
A network engineer working for a private service provider with an employee ID: 1234:56:789 must design a BGP solution based on:
- All traffic originating from AS100 must pass through AS200 to reach the NTP and DHCP server
- When a link failure occurs between R3 and R4, traffic must follow the R2-R9 link to reach the NTP and DHCP server
Which solution must the design include?
A. Routers R3 and R10 advertise an IGP metric into BGP during redistribution in both directions
B. Router R6 influences the paths of R9 and R11 to the DC with a higher AS-PATH value
C. Routers R3 and R10 advertise a lower local preference for outgoing traffic and a higher AS-PATH value for incoming traffic
D. Router R3 applies a local preference of 200 for R1, R2, R9, and R11 routers to reach the data center
Answer
B. Router R6 influences the paths of R9 and R11 to the DC with a higher AS-PATH value
Explanation
Let’s break down the requirements and explain why this solution is the most appropriate:
- Traffic from AS100 must pass through AS200 to reach NTP and DHCP servers:
This requires manipulation of BGP path selection to prefer the route through AS200. - In case of R3-R4 link failure, traffic should use the R2-R9 link:
This necessitates a backup path that’s less preferred under normal conditions but becomes the best path when the primary fails.
Option B achieves both requirements by having R6 in AS200 influence the path selection:
- By increasing the AS-PATH length for routes advertised from R6 to R9 and R11, it makes the path through AS200 less preferred for traffic originating from AS100.
- This configuration ensures that under normal conditions, traffic will prefer the path R1/R2 -> R3 -> R4 -> Data Center.
- If the R3-R4 link fails, the longer but still available path through R2 -> R9 -> R10 -> R6 -> R5 -> R4 -> Data Center becomes the best path.
The other options are less suitable:
- A: IGP metric redistribution doesn’t guarantee the desired path selection across different AS.
- C: Manipulating local preference and AS-PATH on R3 and R10 doesn’t ensure traffic passes through AS200.
- D: Setting a high local preference on R3 for routes to the data center contradicts the requirement to pass through AS200.
This solution provides the required traffic flow under normal conditions and the desired failover behavior, making it the most appropriate design for the given scenario.
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