What is tier in the first place ? If you are dealing with Service Provider networks, you hear this term a lot. But how we define Tier 1,Tier 2 and Tier 3 Service Providers ?
What should be their infrastructure to be seen as Tier 1 for example ?
Which tier is bigger in scale ? Which one is better for the customers to purchase a service from ?
Why Service Providers claim that they are Tier 1 or Tier 2 ?
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Let’s start with the definition first.
Tier 1 Service Provider : A network, which does not purchase transit service from any other network, and therefore peers with every other Tier 1 network to maintain global reachability. They are the biggest guys geographically, but not always the number of customers point of view.
Tier 2 Service Provider : A network with transit connections , customers and some peering, but that still buys transit service from Tier 1 Providers to reach some portion of the Internet.
Tier 3 Service Provider : A stub network, typically without any transit customers, and without any peering relationships. They generally purchase transit Internet connection from Tier 2 Service Providers, sometimes even from the Tier 1 Providers as well (I know some non-profit organisations which have a transit connection from Tier 1)
Tier 1, Tier 2 and Tier 3 Service Providers
Above picture shows the general idea behind Tier 1,Tier 2 and Tier 3 Service Providers connections and relationship. Tier 2 Providers generally can be peer with another Tier 2 and Tier 1 Service Providers only peer with other Tier 1.
Logic behind is actually very simple.
Tier 1 Service Providers don’t peer with Tier 2 because, Tier 2 providers are potential customers of Tier 1 Service Providers. If they can be a customer and pay the money for the transit connection, why would give them a peer connectivity (Peering is free, at least in theory)
Unless the customer changes their path preference with communities, service providers almost always choose customer over peering links vs. transit links.
They want to utilize the customer links, because they pay for the transit service. Even though peering is free thus SPs don’t pay each other for the service, peering brings them some cost. (They need to have a connection to the IX and have a router and port in the IX).
There are just 11 or 12 Tier 1 Service Provider in the world and some Tier 2 level Service Providers always claim that they are Tier 1. By doing it, they target to have a free peering relation with the other Tier 1 of course so they wouldn’t pay transit cost and have other Tier 2 SPs as their customers.
Same thing is valid for the Tier 3 Service Providers as well. They might try to show them as Tier 2 to get free peering from the other Tier 2 Service Providers. But often the Service Providers put strict requirements for the peering so claiming may not help !
Last but not least, some thoughts for my more advanced readers; if an ISP is Tier 1 for IPv4, is it also Tier for IPv6 ? We can discuss in the comments or if you want to read my thoughts in detail let me know in the comment section below, so I write a separate article and we discuss there.
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