Link Aggregation: Death knell for FDDI
 
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Just a few years ago customers had many compelling reasons to deploy FDDI as a backbone technology in preference to Fast Ethernet. Although both topologies operate at 100M bit/sec, FDDI promised some significant advantages over Fast Ethernet, albeit at a higher price. Link Aggregation stands to change that by empowering Fast Ethernet with many of the features that were once the exclusive domain of FDDI. During the next three weeks, we'll compare some specific features of Fast Ethernet Link Aggregation and FDDI and see how each one stacks up. We'll start this week by looking at FDDI's dual attachment option and consider what it offers, and what it doesn't.
One of the main advantages of FDDI is the built-in fault tolerance of dual attachment configuration. In a dual attachment FDDI network, the ring can bypass the fault in a few milliseconds or less. Unfortunately, dual attachment requires a half-duplex connection. That means that all devices on the LAN must compete for the available 100M bit/sec bandwidth. Therefore, even in a switched FDDI connection, the two physical cables that connect neighboring devices provide only a single 100M bit/sec connection.
By contrast, Link Aggregation supports full duplex connections and aggregates the bandwidth of all connections between attached devices. In other words, two full duplex Fast Ethernet links aggregated together could deliver up to 400M bit/sec (200M bit/sec over each full duplex link). Clearly, this is significantly more than the FDDI solution.
Link Aggregation provides fault tolerance, too. Tests conducted by The Tolly Group demonstrated that Link Aggregation solutions recover from link failure is less than three seconds (see The Tolly Group documents 8280, 98256, and 98259 through 98262). Of course that sub-3 second recovery only applies to traffic crossing the failed link, traffic crossing other links remains unaffected.
While the recovery time for Link Aggregation is much slower than that of FDDI, Aggregation does provide the benefit of significantly higher bandwidth whenever the network is operational. Moreover, in most cases, sub-3 second recovery means that Link Aggregation - like FDDI - prevents session loss. For many customers, the benefits of Link Aggregation clearly outweigh its drawbacks.
Next week, we'll compare FDDI's dual homing with Link Aggregation.
RELATED LINKS
Network World Fusion, 4/10/98
Bridging IPX between token ring and FDDI or Ethernet
Network World, 3/23/98
