This is a silly(maybe interesting) question.

Assume four ASes are connecting to the Tier 1 network(A, B, C and D). B is peering with C and D. A is close to B, but too far from C and D to make peering. If A wants to connect to C and D, data is usually transferred through the backbone(Tier 1 network).

Do we have the option of B as a transit(A->B->C, D), bypassing the backbone? We usually only mention doing peering in the same-level networks(IXP). So I don't know if this idea makes sense.

  • There is no formal tier definition, and ASes are not forced into connecting to any other AS.. One AS peers with one or more other ASes as it wants and can get contracts. Some contracts may allow transit services, while others may forbid it. Also, there is no distance factor with ASes. Many ASes are all across the world. Distance refers to AS hops, but any AS can peer with any other AS.
    – Ron Maupin
    Feb 2, 2023 at 13:27

2 Answers 2


This all depends on how these networks are connected. If there is no direct BGP connection between A and B, this would never work.

Your assumptions on what 'peering' and 'transit' are, are a bit off I think. Mostly, these concepts refer to the financial relationship between networks. 'peering' usually means it's settlement-free, while 'transit' is paid. What routes are exchanged can differ for both types. 'paid peering' (where one network receives routes from another network and its downstream networks) and 'partial transit' (where a network receives a specific subset of the routes in the global routing table) are two other variants of relationships between two networks.

In this specific case:

Network B could decide to announce C and D's prefixes to A, and A can choose to accept them. But there's usually no real motivation to do so. Network B would allow traffic from another to another network to flow through their network, consuming resources and capacity, without any financial compensation. So B could ask A to pay for receiving the routes and transporting the traffic to C and D, making it a form of partial transit.

  • Thanks for your answering again! I thought the routing relationship between ASes is just peering or transit. Now it seems that peering and transit are general terms for some common types of connections in business, like IXP. Not each type of routing policy is strictly defined by a specific term. Could you please tell me some other common variants of relationships? I only have a few theoretical understandings of BGP.
    – wyb
    Feb 2, 2023 at 10:37

Technically it's definitely possible, it all depends on B's configuration. How they route traffic between locations and whether they permit transit traffic is totally up to them.

  • Is there a specific term to describe this relationship between A and B? I only find transit and peering to describe the AS relationship, but I don't think this case is transit or peering.
    – wyb
    Feb 2, 2023 at 7:51
  • 1
    I've heard this arrangement, where b provides a with access to it's peers, but not access to the internet as a whole referred to as "partial transit". Feb 3, 2023 at 16:45

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