Click the exhibit. 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 the ISPs?

Question

Options

• AISP 1 and ISP 2 have a public transit relationship, while ISP 3 and ISP 4 have a private transit relationship.
• BISP 1 and ISP 2 have a private transit relationship, while ISP 3 and ISP 4 have a public transit relationship.
• CISP 1 and ISP 2 have a private peering relationship, while ISP 3 and ISP 4 have a public peering relationship.
• DISP 1 and ISP 2 have a public peering relationship, while ISP 3 and ISP 4 have a private peering relationship.

• A
  8% (3)
• B
  3% (1)
• C
  16% (6)
• D
  74% (28)

Explanation

D is correct because Tier 2 ISPs typically peer with each other at Internet Exchange Points (IXPs) - shared public facilities - making their relationship public peering, while Tier 1 ISPs, which carry massive traffic volumes and have the resources for dedicated infrastructure, establish direct point-to-point connections with each other, making their relationship private peering.

Options A and B are wrong because both describe transit relationships, which imply a paid customer-provider hierarchy. Peering between ISPs at the same tier is generally settlement-free (neither pays the other), so "transit" doesn't apply here at all.

Option C is wrong because it reverses the logic - it assigns private peering to Tier 2 and public peering to Tier 1, which is backwards given that Tier 1 providers have both the traffic volume and the business incentive to justify costly private interconnects, while smaller Tier 2 providers share the economies of a public IXP.

Memory tip: Think "size = privacy" - the bigger the ISP (Tier 1), the more likely they invest in private direct links; the smaller ISP (Tier 2) shares a public exchange point to keep costs down.

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