Link Aggregation vs. FDDI Dual Homing

Last week we compared Fast Ethernet Link Aggregation with FDDI dual attachment. We noted that even though Link Aggregation does not recover from a link failure quite as fast as FDDI, Link Aggregation does recover in less than three seconds which is more than adequate for many customers. Moreover, Link Aggregation delivers many times the throughput of FDDI. This week we will consider how FDDI's other failover technique - dual homing - stacks up against Link Aggregation.

FDDI Dual Homing involves a single FDDI device with one "active" connection to the network and one "hot standby." Like dual attachment, dual homing provides very fast recovery from a link failure (less than a few milliseconds). However, like dual attachment, dual homing is generally limited to a half-duplex operation which means is has 100M bit/sec of throughput, at the most, at any time. Furthermore, dual homing only allows for one standby connection.

Compare this with link aggregation. Not only does link aggregation allow multiple full duplex connections, it provides automatic failover to any of the active connections. In other words, in a network of four aggregated Fast Ethernet links, each one of the four links is backed up by the other three. This provides yet another benefit; traffic from the failed link can theoretically be distributed among all of the active links. Thus, more links means better throughput both before and after a failure.

Of course, customers still need to choose between 3-second failover and several hundred M bit/sec of throughput with link aggregation, or 100M bit/sec of throughput and near-instantaneous failover with FDDI. For those customers who want to optimize performance for a fully functional network, link aggregation once again seems a far more attractive option than FDDI.

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