Consider the pictured network implementing BGP. *link for BGP topology https://i

Question

Consider the pictured network implementing BGP.

*link for BGP topology https://ibb.co/kAP4Na

- Nodes evaluate path choices based on AS path lengths (the shorter the better).

- If multiple providers give equally good paths to send outbound traffic, a customer will pick the provider with the highest AS number (can be achieved by using weight/local preference).

- A service provider will not export routes learned from a peer to another peer.

Network operators typically like to equalize the amount of traffic coming from multiple providers. Suppose AS 6 owns prefix 123.45.0.0/16, and all IP addresses in this prefix receive approximately the same amount of traffic.

3. AS 6 constructs an announcement of its prefix 123.45.0.0/16 and sends the same announcement to both AS 3 and AS 4. What will happen to the traffic? Is AS 6 guaranteed to get the same amount of traffic from both providers?

4. Now AS 6 sends different announcements: prefix 123.45.0.0/17 to AS 3 and prefix 123.45.128.0/17 to AS 4. Does this equalize the amount of traffic? What problems do you see in terms of scalability and reliability?

Explanation / Answer

C. The traffic that will route via provider is not under the direct control of customer. Some part of the traffic will route via AS 3 and rest of the part will route via As 4. And as peer doesnot communicate the learned path as well as customer doesn't have the controll over the traffic via provider so there is no garuntee that same ammount of traffic will route via both the provider.

D. Yes with a different but single announcement, the incoming traffic via both the providers can be equalized.

Now If all the AS did this sort of deaggregation, it would double the number or prefixes announced in the Internet, and double the space used in many routers’ forwarding tables. The space usage hamper the scalability of the network.Though in practice, ASes do deaggregate to do traffic engineering in smaller chunks.

Now for reliability if one link fails, the IP prefix announced along that link will become unreachable, even though there is a working path via the other provider.

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