300-320 Question #665: Real Exam Question with Answer & Explanation

The correct answer is D: To load share ingress traffic, one option you have is to AS prepend.. (D) With one router and two ISP uplinks, you can influence ingress (inbound) traffic distribution by using AS path prepending. By prepending your AS number on advertisements sent to one ISP, you make that path appear longer/less attractive, causing remote ASes to prefer the other

Question

Which statement is true about load sharing when you have a single-router network connecting to two ISPs?

Options

AYou can use BGP multipath.
BYou can easily use either PA or PI.
CFor redundancy, advertise routes that you receive from ISP1 to ISP2.
DTo load share ingress traffic, one option you have is to AS prepend.

Explanation

(D) With one router and two ISP uplinks, you can influence ingress (inbound) traffic distribution by using AS path prepending. By prepending your AS number on advertisements sent to one ISP, you make that path appear longer/less attractive, causing remote ASes to prefer the other ISP - effectively shifting inbound traffic ratios. (A) BGP multipath requires equal-cost paths, which is unlikely across two different ISPs. (B) Provider-independent (PI) space is generally required for proper multihoming; PA space is tied to the ISP. (C) Advertising ISP1 routes to ISP2 would make you a transit AS, which is undesirable and likely contractually prohibited.

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