Given that ISP 1 and ISP 2 are Tier 2 providers, and that ISP 3 and ISP 4 are Tier 1 providers, what is the most likely relationship between ISP 1 and the other ISPs?

Question

Options

• AISP 1 has a peering relationship with ISP 2 and ISP 4.
• BISP 1 has a peering relationship with ISP 2 and a transit relationship with ISP 3.
• CISP 1 has a peering relationship with ISP 2 and ISP 3.
• DISP 1 has a transit relationship with ISP 2 and ISP 4.
• EISP 1 has a transit relationship with ISP 2 and a peering relationship with ISP 3.

• A
  2% (1)
• B
  82% (41)
• C
  10% (5)
• D
  2% (1)
• E
  4% (2)

Explanation

Option B is correct because ISPs form relationships based on their tier hierarchy: same-tier ISPs peer with each other (exchanging traffic for free, mutually beneficial), while lower-tier ISPs pay higher-tier ISPs for transit to reach the rest of the internet. Since ISP 1 and ISP 2 are both Tier 2, they peer; since ISP 3 is Tier 1 (above ISP 1), ISP 1 pays ISP 3 for transit.

Why the distractors fail:

• A and C correctly identify the peering relationship with ISP 2, but wrongly claim ISP 1 peers with a Tier 1 provider (ISP 4 or ISP 3) - Tier 1s don't peer with Tier 2s; they sell them transit.
• D incorrectly says ISP 1 has a transit relationship with ISP 2 - two Tier 2 ISPs peer with each other, they don't pay one another.
• E flips both relationships backwards: ISP 1 would not pay transit to a same-tier ISP 2, nor peer with a higher-tier ISP 3.

Memory tip: Think of internet tiers like a corporate hierarchy - you collaborate as equals with coworkers (peering), but you pay your boss for resources (transit). Same tier = peer; climbing the tier = pay for transit.

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