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126 Summit 300-48 Switch Software User Guide
Quality of Service (QoS)
Explicit Class of Service (802.1p and DiffServ) Traffic Groupings
This category of traffic groupings describes what is sometimes referred to as explicit packet marking, and
refers to information contained within a packet intended to explicitly determine a class of service. That
information includes:
• IP DiffServ code points, formerly known as IP TOS bits
• Prioritization bits used in IEEE 802.1p packets
An advantage of explicit packet marking is that the class of service information can be carried
throughout the network infrastructure, without repeating what can be complex traffic grouping policies
at each switch location. Another advantage is that end stations can perform their own packet marking
on an application-specific basis. The Summit 300-48 switch has the capability of observing and
manipulating packet marking information with no performance penalty.
The documented capabilities for 802.1p priority markings or DiffServ capabilities are not impacted by
the switching or routing configuration of the switch. For example, 802.1p information can be preserved
across a routed switch boundary and DiffServ code points can be observed or overwritten across a
layer 2 switch boundary.
NOTE
Re-marking DiffServ code points is supported through access lists. See Chapter 10, “Access Policies”,
for more information.
Configuring 802.1p Priority
Extreme switches support the standard 802.1p priority bits that are part of a tagged Ethernet packet.
The 802.1p bits can be used to prioritize the packet, and assign it to a particular QoS profile.
When a packet arrives at the switch, the switch examines the 802.1p priority field maps it to a specific
hardware queue when subsequently transmitting the packet. The 802.1p priority field is located directly
following the 802.1Q type field, and preceding the 802.1Q VLAN ID, as shown in Figure 13.
Figure 13: Ethernet packet encapsulation
Observing 802.1p Information
When ingress traffic that contains 802.1p prioritization information is detected by the switch, the traffic
is mapped to various hardware queues on the egress port of the switch. The Summit 300-48 switch
EW_024
Source
address
802.1Q
type
802.1p
priority
802.1Q
VLAN ID
IP packet CRC
Destination
address
8100