Weidmuller IE-SW-VL08M Series User Manual page 59

IE-SW-VL08M Series User's Manual
DSCP is backward compatible with IPV4 TOS, which allows operation with existing devices
that use a layer 3 TOS enabled prioritization scheme.
Traffic Prioritization
The IE-SW-VL08M classifies traffic based on layer 2 of the OSI 7 layer model, and the switch
prioritizes received traffic according to the priority information defined in the received packet.
Incoming traffic is classified based upon the IEEE 802.1D frame and is assigned to the appropriate
priority queue based on the IEEE 802.1p service level value defined in that packet. Service level
markings (values) are defined in the IEEE 802.1Q 4-byte tag, and consequently traffic will only
contain 802.1p priority markings if the network is configured with VLANs and VLAN tagging. The
traffic flow through the switch is as follows:
1. A packet received by the switch may or may not have an 802.1p tag associated with it. If it does
not, then it is given a default 802.1p tag (which is usually 0). Alternatively, the packet may be
marked with a new 802.1p value, which will result in all knowledge of the old 802.1p tag being
lost.
2. Because the 802.1p priority levels are fixed to the traffic queues, the packet will be placed in the
appropriate priority queue, ready for transmission through the appropriate egress port. When the
packet reaches the head of its queue and is about to be transmitted, the device determines
whether or not the egress port is tagged for that VLAN. If it is, then the new 802.1p tag is used
in the extended 802.1D header.
The switch will check a packet received at the ingress port for IEEE 802.1D traffic classification,
and then prioritize it based upon the IEEE 802.1p value (service levels) in that tag. It is this 802.1p
value that determines which traffic queue the packet is mapped to.
Traffic Queues
The IE-Sw-VL08M hardware has multiple traffic queues that allow packet prioritization to occur.
Higher priority traffic can pass through the device without being delayed by lower priority traffic.
As each packet arrives in the device, it passes through any ingress processing (which includes
classification, marking/re-marking), and is then sorted into the appropriate queue. The switch then
forwards packets from each queue.
The IE-SW-VL08M supports two different queuing mechanisms:
Weight Fair: This method services all the traffic queues, giving priority to the higher priority
queues. Under most circumstances, this method gives high priority precedence over
low-priority, but in the event that high-priority traffic exceeds the link capacity, lower priority
traffic is not blocked.
Strict: This method services high traffic queues first; low priority queues are delayed until no
more high priority data needs to be sent. This method always gives precedence to high priority
over low-priority.
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