Optimizing access networks with Layer 3 switching

What went around has come around again. As Ethernet-based technology continues its advance into access networks, network operators and equipment vendors are facing many of the same decisions that once concerned enterprise and carrier networks. One in particular is whether to base a network on Layer 2 or Layer 3 switching.

Layer 2 switching offers the temptations of easy operation and maintenance. However, just as was the case for enterprise and carrier networks, those temptations are illusions. In fact, Layer 3 architectures are easier to manage, provide better performance and allow significantly greater scalability. The bottom line is that Layer 3 switching creates the optimum architecture for access networks.

Commercial networks must be managed networks, whatever their underlying architecture. Although Layer 2 and Layer 3 networks require the same amount of IP infrastructure and planning, only Layer 3 architectures can take advantage of automated tools to ease deployment. Only Layer 3 architectures provide nearly unlimited flexibility in mixing media and network technologies; and only Layer 3 switching provides built-in isolation between network segments to make troubleshooting and fault management simpler.

Some Layer 2 proponents claim that its architectures are superior because they don't require planning for and establishing an IP infrastructure, including addressing assignments and subnetwork boundaries. But the fact is any network that delivers IP services requires an IP infrastructure. To the extent that Layer 3 architectures require operators to plan and engineer their networks, those architectures ultimately result in superior networks and services.

Once network deployments move beyond the planning stage and into actual operation, the benefits of Layer 3 architectures become clear. Layer 3 technology based on IP is widely deployed by enterprises and carriers. With such a large market to serve, engineers have developed powerful and convenient tools to automate and simplify network configuration and operation, tools that are lacking in Layer 2 environments.

Consider how the two architectures distinguish network elements. In Layer 3 networks, the primary identifier is an IP address. Network operators have complete flexibility in assigning these IP addresses to the various systems, so they can assign them in a manner that most appropriately matches their deployment strategy.

Contrast the flexibility and automation available with IP addressing to that of the Layer 2 system identifier: the media access control (MAC) address. Equipment manufacturers, not network operators, assign MAC addresses, leaving operators no control over the mapping. Unlike an IP address, a device's MAC address often changes when the device is repaired or replaced, significantly increasing the burden on an operator's actively maintaining its network.

Similar problems face operators attempting to logically segment their Layer 2 networks. While Layer 3 networks are naturally segmented by IP subnetworks without any special work on the operator's part, the Layer 2 equivalent, a virtual LAN, must be manually configured in each device.

Another significant advantage of Layer 3 architectures is their inherent support for multiple network technologies. Layer 2 networks are, by definition, confined to a particular Layer 2 technology such as Ethernet or ATM. Layer 3 architectures, on the other hand, can encompass all network technologies and still provide a common management and operational infrastructure.

Troubleshooting is also easier when problems are easier to isolate. Layer 3 architectures consist of many connected subnetworks so they have isolation inherent in their construction. Routers that interconnect subnetworks ensure that only appropriate, Layer 3 traffic passes from one subnetwork to another. Layer 2 networks do not provide this isolation.

Isolation is critical when malfunctioning systems generate broadcast packets, especially packets that elicit responses that are also broadcast, a scenario known as a broadcast storm that can be devastating to a network. Often, the only way to recover is to completely power off all devices on the network. A Layer 2 architecture provides no protection against broadcast storms, the entire access network serving all subscribers would be affected. Layer 3 switches, on the other hand, provide built-in isolation; broadcast storms are limited to a single subscriber.

- Stephen Thomas

Chief Architect

Wave7 Optics Inc.