Forward Unknown: By default, messages addressed to unicast addresses that have not
yet been learned by the switch are flooded to all other ports. This is important for some
protocols that transfer data primarily in one direction, but it can overwhelm smaller systems
that do not expect a large amount of traffic. Forwarding of unknown unicast messages can be
disabled on a port-by-port basis by disabling “Forward Unknown”.
QoS Example
Let us investigate a detailed example of how to manage a network such that critical real time
data will not be interrupted by data that is not as urgent
Hypothetical Scenario:
There is a power plant that is controlled by a central control system. In addition, because of
security concerns, cameras have been mounted and installed at each location of mechanical
control. The mechanical control devices and video cameras at each site communicate via
Ethernet to their own switch. (For reasons of simplicity and clarity, we will assume that only
video and control data reside on the network).
• Problem: Should any of the mechanical control devices receive delayed control data from
the central control system, the power plant can’t generate the maximum energy that it
is capable of. Customers will experience brown outs, and the plant will be looked upon
with negative scrutiny. It is therefore very important that the video traffic created by the
cameras not delay critical data.
• Goal: To optimize the forwarding of critical real-time control data and minimize or
eliminate the impact of video data traversing the network.
• Solution: Configure the switch such that video data has lower priority than control data
by adjusting the priority queuing settings in the switch.
Configuration of the Switch:
As mentioned earlier in this manual, some applications require a certain Quality of Service
(QoS) from the network to achieve a desired level of service. In this example, it is important
that we achieve timeliness for control data. Without taking advantage of the switch’s priority
queuing abilities, we are using the best-effort network model. This means that the network
will try to deliver all packets of information, but will not make any sort of promise or
guarantees with respect to the timeliness of data for specific applications. Considering our
control/video example, there is no guarantee that we can get the response time needed for
control data if the video cameras are sending data at the same time.
A way to achieve the QoS desired is to prioritize network traffic. Prioritization of network
traffic can be achieved even if the devices (video cameras and control systems) do not support
selection or configuration of Quality of Service parameters.
Configure all the ports used to interconnect the switches as follows:
• Use 802.1p Tag Priority = Checked
• Use IP ToS/DiffServ = Checked
• Default Priority Precedence = Tag
• Output Tag = Add Tag
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Chapter 4 - Managed Switch Software Setup
Stride Industrial Ethernet Switches User Manual 2nd Ed. Rev. A
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