300-320 Question #503: Real Exam Question with Answer & Explanation

The correct answer is A: accept all routes from ISPs inbound. When dual-homed to two ISPs, your AS can inadvertently become a transit AS if you re-advertise routes learned from ISP-A to ISP-B and vice versa. To prevent this: (A) Accepting all routes inbound from each ISP gives you full routing visibility for your own forwarding decisions -

Question

You use 2x ISPs for the internet connectivity. How could you avoid your internal network to become a transit area (Choose two)?

Options

Aaccept all routes from ISPs inbound
Badvertise all routes outbound
Cfilter internal routes inbound
Dfilter internal routes outbound
Euse just one ISP

Explanation

When dual-homed to two ISPs, your AS can inadvertently become a transit AS if you re-advertise routes learned from ISP-A to ISP-B and vice versa. To prevent this: (A) Accepting all routes inbound from each ISP gives you full routing visibility for your own forwarding decisions - this alone does not make you a transit. (D) Filtering what you advertise outbound ensures you only announce your own prefixes (your address space) to each ISP, and never re-advertise routes you learned from the other ISP. Without outbound filtering, you could forward transit traffic between the two ISPs, consuming your bandwidth and violating ISP policies.

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