Installation guide
Network Infrastructure for EtherNet/IP™
Designing the Infrastructure
4-50
4.9.1 QoS Flow
The common definition of QoS in Layer 2 switches is to prioritize native encapsulated Ethernet frames or
to honor 802.1p Class of Service (CoS) tagged Ethernet frames.
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However, advanced QoS mechanisms
take this definition a step further. Layer 2 QoS on advanced Ethernet switches entail up to four distinct
stages: classification, policing, marking, and queue/schedule. As shown in Figure 4-10, the first three
represent actions taken at ingress, and queue/schedule is an action taken at egress.
Figure 4-10 Quality of Service Stages.
Feature-rich switches can be used to study and interpret the flow of QoS traffic through the switch from
ingress, as it gets processed through the switch fabric, and as it flows out (egress).
A switch can be configured to prioritize frames based on given criteria at different layers of the OSI
Model’s stack. For example, priorities may be assigned according to the source MAC Address (Layer 2)
or the destination TCP port (Layer 4). Any traffic traveling through the interface to which these criteria
apply will be classified and tagged within the frame header, assigning them a given priority. Once the
packet has been classified, it can be policed/marked down (if necessary), and it will be queued according
to the specified priority. Once it has been placed into the appropriate holding queue for transmission on
egress, it will be scheduled based on the configuration of the scheduling algorithm.
4.9.2 Class of Service
Class of Service (CoS) refers to three bits on an 802.1Q header that are used to indicate the priority of the
Ethernet frame as it passes through a switch network. The CoS bits in the 802.1Q header are commonly
referred to as the 802.1p bits. Not surprisingly, there are three CoS bits that match the number of bits used
for IP Precedence (which is part of the Type of Service [ToS] byte). In many networks, to maintain end-
to-end QoS, a packet may traverse both Layer 2 and Layer 3 domains, thus the ToS and CoS fields should
be mapped to each other.
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IEEE Std 802.1p is a Standard used in IEEE Std 802.1D, Standard for Local and Metropolitan Area Networks: Media Access
Control Bridges, and used in IEEE Std 802.1Q, IEEE Standard for Local and Metropolitan Area Networks – Virtual Bridged
Local Area Networks. http://standards.ieee.org/getieee802/802.1.html
Classify packet
based on priority
or flag.
Determine if packet
is in profile or out of
profile based on
policer associated
with filter.
Based on whether packet
is in or out of profile and
configured parameters,
determine forwarding
action and act
accordingly.
Based on CoS,
determine into which of
the egress queues to
place packet. Then
service queues
according to configured
weights.
Classification
Policing
Marking
Queue/
Schedule
Actions
at Ingress
Actions
at Egress