Information Technology Reference
In-Depth Information
Transit and Peering Overview
The focus so far has been on the ISP infrastructure and downstream connectivity for
customer connections. This section discusses upstream ISP connectivity to the rest of the
Internet. There are three primary types of upstream connectivity:
•
Transit
•
Peering—public and private
•
ISP tiers and peering
The subject of upstream connectivity is one of the most political aspects of the ISP
business, as you'll learn in the next sections.
Transit Connectivity
Transit connectivity is the most common form of connectivity available to small-to-
medium-sized ISPs. Transit service essentially means buying full connectivity to the
Internet from another ISP. An ISP sells transit service to its end customers. A transit
connection means that the upstream provider lets the customer transit its network to reach
any available destination on the Internet.
Peering
The term
peering
refers to both public and private peering. Peering in a general sense
between two ISPs means that reachability to that ISP and its direct customers is provided
over that connection. If ISP 1 and ISP 2 initiate a peering connection, they can reach each
other but not ISP 3 if it is not a customer of either ISP 1 or ISP 2. Essentially, peering
involves the exchange of partial routes between the two peering ISPs. The cost of peering
is typically less than full transit service, because both ISPs peering expect to offload the
traffic passing between their customers from their transit links.
Public Peering
Public peering occurs at one of the public peering points, such as Network Access Points
(NAPs), Metropolitan Area Exchanges (MAEs), or Internet Exchange Points (IXPs). Typ-
ically, peering at a public peering point is done over a broadcast medium, such as Fast
Ethernet or Gigabit Ethernet. Several of the major exchanges have started offering ATM
service at the exchange points to provide quality of service (QoS) guarantees. An ISP
