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Web OS 10.0 Application Guide
Statistical Load Distribution
Network 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 information
present in each transmitted frame for determining load distribution.
The addition of Layer 3 IP address examination is an important advance for traffic distribution
in trunk groups. In some port trunking systems, only Layer 2 MAC addresses are considered in
the distribution algorithm. Each packet's particular combination of source and destination
MAC addresses results in selecting one line in the trunk group for data transmission. If there
are enough Layer 2 devices feeding the trunk lines, then traffic distribution becomes relatively
even. 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 MAC
address combinations encountered results in a lopsided traffic distribution, which can reduce
the effective combined bandwidth of the trunked ports.
By adding Layer 3 IP address information to the distribution algorithm, a far wider variety of
address combinations is seen. Even with just a few routers feeding the trunk, the normal
source/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 combined
bandwidth available to trunked ports.
Built-In Fault Tolerance
Since each trunk group is comprised of multiple physical links, the trunk group is inherently
fault tolerant. As long as one connection between the switches is available, the trunk remains
active.
Statistical load balancing is maintained whenever a port in a trunk group is lost or returned to
service.
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Chapter 3: Port Trunking
212777-A, February 2002
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