4.1WAN and wide-area connectivity

For a network designer at a multi-site company, choosing a site-to-site connectivity method is a three-way tradeoff among cost, quality (bandwidth guarantee and latency stability), and security. Options range from a leased line—high quality and high security but expensive—to an Internet VPN, which is cheap but offers weak quality guarantees. The optimal choice shifts with requirements. This section builds the judgment needed to decide which WAN service to choose given concrete constraints such as site count, required quality, and budget.

4.1.1Leased lines and wide-area Ethernet

• A leased line is a circuit dedicated to a single subscriber on the carrier's facilities. Because it is not shared with other customers' traffic, bandwidth and latency are fully guaranteed and confidentiality is highest, but its chief drawback is that cost rises with distance (a full-mesh topology across many sites becomes prohibitively expensive).
• Wide-area Ethernet transparently carries Ethernet frames across the carrier's network, connecting sites at Layer 2. Even with many sites, hub-and-spoke or full-mesh topologies can be configured flexibly, and cost does not scale as sharply with distance as a leased line does. Because it is an L2 connection, it also offers leased-line-like freedom to design the IP addressing scheme across sites.