Building Fault-Tolerant Ethernet Networks

By Joel Conover  Mission-critical networks demand 24x7 reliability. In a work environment where one minute of downtime can mean tens of thousands of dollars in lost sales and productivity, you can't afford to have a network failure. Although network topologies like FDDI and ATM offer fault-tolerant network connections, most mission-critical networks are built on Ethernet and Fast Ethernet backbones, which do not. Fortunately, vendors are responding with innovative ways to add fault tolerance to these networks. The Spanning Tree algorithm is the most universal method of provisioning multiple paths through an Ethernet network. Unfortunately, it is cumbersome and slow to converge. Newer technol ogies offer multiple active paths through the network, improving performance and adding resiliency at the same time. However, your choices are limited and you will pay a premium--hundreds to millions of dollars depending on the reach of your resilient network. Most of these technologies are proprietary, though some will work with existing hardware. We've tested many of these solutions in Network Computing's University of Wisconsin lab. Some technologies, like Adaptec's DuraLink software and Performance Technologies' Nebula 8000, offer a standards-based approach to fault tolerance. Others, like products which utilize Cisco Systems' Fast EtherChannel technology, are proprietary today. But the road is paved for Fast EtherChannel to become a legitimate standard. Other technologies, such as Cabletron Systems' SecureFast networking, will remain totally proprietary. And whether gigabit trunking becomes a universal standard is still to be determined. Adding fault tolerance to an existing network will be a bigge r challenge than building a new fault-tolerant design. Adaptec and Intel Corp. offer solutions that you can drop into your existing network to add resiliency. Fast EtherChannel offers a very scalable and resilient design, and it is gaining momentum in the market. Other technologies presented here are proprietary and vary in cost and complexity. Price will likely have a large impact on your purchasing strategy if you're bidding for a new network. In making your decision, give equal weight to the longevity of the technology you'll be saddling up with. Server Resiliency Traditionally, resilient server connections have been relegated to the world of FDDI. However, a new breed of Fast Ethernet server adapters promises to break that mold. Products from Adaptec, Intel, LANart Corp. and ZNYX Corp. offer products for Fast Ethernet. Some even offer improved bandwidth using multiple active channels. · Adaptec DuraLink Failover Adaptec's DuraLink Failover software, a free add-on for Adaptec (and Cogen t) interface card owners, lets you connect multiple Adaptec Fast Ethernet ports to a single switch. It accomplishes this independent of the switching hardware, making it the only 100 percent interoperable solution we tested. But DuraLink doesn't stop at failover. With the optional DuraLink Port Aggregation software, you can have multiple active connections to a single switch, letting you load-balance traffic across multiple Ethernet connections. We tested both the DuraLink Failover and DuraLink Port Aggregation drivers. Both varieties support failover should one or more ports fail. In our test setup, we installed an Adaptec ANA-6944A four-port 10/100 in a four-way Pentium Pro Compaq ProLiant server running Microsoft Windows NT 4.0. Configuring the failover drivers was child's play. You can configure the drivers to detect network failures in increments of one second. With 16 clients beating on the server, we disconnected first one, then two and three of the four connections to our Cisco Catalyst 2926 switch . Each time, the clients recovered in less than three seconds. When the cable is reconnected, existing traffic does not return to the restored segment. However, as new clients connect to the server, the DuraLink software will balance the new connections onto the restored segments.

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