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Layer 3 Switches: Still The Gold Standard

Friday, March 4th, 2016 by

l2swLayer 3 switches can slash router latency, but the current lack of unified industry standards necessitates cautious deployment of the technology

Reducing router latency is one of the top priorities of many network managers, so the industry’s crazed rush to deliver Layer 3 switching technology is not surprising. Unfortunately, little agreement has been reached on a unified Layer 3 switch design, and it doesn’t appear that standards are looming in the wings.

Because of this lack of uniformity, we urge network administrators to take only cautious steps toward implementing this new technology. Those desperate to deploy Layer 3 switches should start from the outer edges of the network, rather than overhauling the backbone with products that might not fully interoperate with installed infrastructures or future standards.

Detouring around congestion

Layer 3 switch technology can substantially reduce router latency and turbocharge communications. A logical extension to LAN switching, Layer 3 switching delivers routing capabilities–which normally occur at the OSI Network layer–at the lower latencies associated with OSI Layer 2 (the Data Link layer) switches.

When network congestion in shared-bandwidth LANs prompted corporations to adopt LAN switching, the conventional routers in backbones and the outer edges of networks remained unchanged. Meanwhile, because routers simply cannot keep pace with switches, network traffic increased, causing a new bottleneck. Thus, enter Layer 3 switching.

Switches and routers operate at different OSI layers. Switches function in Layer 2, whereas routers operate at Layer 3 (the Network layer). Responsible for providing a packet-transfer service, Layer 3 manages a variety of information within a protocol stack, such as TCP/IP. Layer 2 switches, in contrast, are simple frame-forwarding devices and only use the MAC (media access control) addresses to choose a frame’s destination, so they require less overhead to read and manipulate each packet–switches that operate at Layer 2 are much faster than Layer 3-based routing. Because they include Layer 3 routing intelligence in the Layer 2 switch’s hardware, Layer 3 switches eliminate the usual bottleneck associated with routing between subnets.

In the context of VLANs (virtual LANs), Layer 3 switching allows network managers to define broadcast domains within a switched LAN using specific protocol criteria, such as an IP subnet or an IPX network number. Other forms of creating VLANs include port-based and MAC address-based approaches. Although VLAN specs are defined by IEEE (Institute of Electrical and Electronics Engineers) working group 802.1Q, the IEEE 802 committee is only concerned with the first two layers of the OSI stack, so the Layer 3 VLANs remain proprietary. Therefore, 802.1Q-compliant products follow specifications only on port-based and MAC address-based VLANs.

What’s on the shelves so far?

A number of vendors have shipped Layer 3 switches or are planning to do so in the near future.

But not only are their designs proprietary, there’s not even a standard naming convention: Vendors use terms such as “routing switches,” “multilayer switches” and “Layer 3 switches.” To add to the confusion, vendors are taking the liberty of describing their switches as everything from network-layer-based VLANs to full IP and IPX super routers.

The lack of industry standards and seemingly subtle design variations has translated into vast differences in product functionality.

Among the vendors offering Layer 3 switches are Xylan Corp., Bay Networks Inc., Madge Networks Inc. and Foundry Networks Inc. Most of these switches currently perform IP, IPX, or both IP and IPX routing.

Some Layer 3 switches implement Layer 3 forwarding in software, which allows the switch to be updated to support whatever Layer 3 switching standards become implemented. The majority use ASICs (application-specific integrated circuits) to implement Layer 3 intelligence in the hardware, casting the switch’s Layer 3 implementation in (silicon-based) stone.

Because most of the switches on the market today are based on proprietary ASICs, each switch design is inherently different, making interoperability among different Layer 3 switches impossible.

In fact, some vendors achieve Layer 3 switching by replacing traditional routers and switches at both the core and outer edges of the network with their proprietary solutions. Others, such as Cisco Systems Inc., try to leverage their market share and steer the industry toward accepting their proprietary scheme as the definitive standard.

Layer 3 switches need to interoperate with such network layer protocols as IP. Switches that incorporate standard routing protocols, such as RIP (Routing Information Protocol) and OSPF (Open Shortest Path First), can advertise themselves as routers to end stations and other routers in the network, and can therefore coexist within an infrastructure.

This type of Layer 3 switch–especially when deployed at the outer edges of networks–can coexist with current routers and end stations, although interoperability with other Layer 3 switches from different vendors is not guaranteed.

Network administrators must determine what type of Layer 3 switching–ranging from protocol-based VLAN definitions to full Layer 3 routing capabilities–they need to implement. Emerging standards, such as the 802.1Q specification for VLANs, must be closely monitored, and potential buyers should know whether hardware changes or software augmentations will be necessary to accommodate those upcoming standards.

Perhaps most importantly, network managers should understand whether their current infrastructure devices need to be replaced or reconfigured to interoperate with the new Layer 3 switches.

On the bright side, most Layer 3 switches can provide Layer 2 functionality. Thus, paying a few extra bucks per port (compared with Layer 2 switch prices) is not as risky as it sounds, because you can always use these boxes as traditional Layer 2 switches.

Beyond the up-front cash outlay, a Layer 3 switch is inherently more complex to configure and manage than a traditional router–at least initially. Be prepared to make a large investment in equipment and personnel training.

Although the corporate move toward IP as the main transport protocol coupled with the lure of reduced router latency promised by Layer 3 switch technology is tempting many network professionals to jump onto the Layer 3 switch bandwagon, we urge a cautious approach to current offerings: The importance of distinguishing facts from vendor hype could be the best approach for your network’s health.

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