Web OS 10.0 Application Guide66 n Chapter 3: Port Trunking 212777-A, February 2002Statistical Load DistributionNetwork traffic is statistically load balanced between the ports in a trunk group. The Web OS-powered switch uses both the Layer 2 MAC address and Layer 3 IP address informationpresent in each transmitted frame for determining load distribution.The addition of Layer 3 IP address examination is an important advance for traffic distributionin trunk groups. In some port trunking systems, only Layer 2 MAC addresses are considered inthe distribution algorithm. Each packet’s particular combination of source and destinationMAC addresses results in selecting one line in the trunk group for data transmission. If thereare enough Layer 2 devices feeding the trunk lines, then traffic distribution becomes relativelyeven. In some topologies, however, only a limited number of Layer 2 devices (such as a hand-ful of routers and servers) feed the trunk lines. When this occurs, the limited number of MACaddress combinations encountered results in a lopsided traffic distribution, which can reducethe effective combined bandwidth of the trunked ports.By adding Layer 3 IP address information to the distribution algorithm, a far wider variety ofaddress combinations is seen. Even with just a few routers feeding the trunk, the normalsource/destination IP address combinations (even within a single LAN) can be widely varied.This results in a wider statistical load distribution and maximizes the use of the combinedbandwidth available to trunked ports.Built-In Fault ToleranceSince each trunk group is comprised of multiple physical links, the trunk group is inherentlyfault tolerant. As long as one connection between the switches is available, the trunk remainsactive.Statistical load balancing is maintained whenever a port in a trunk group is lost or returned toservice.
