58 Chapter 2: End-to-End QoS: Quality of Service at Layer 3 and Layer 2
This example reserves 56 kbps of bandwidth for the LLQ, which means that, under
congestion, there will also be a policer that limits the VOICE class to 56 kbps. Again, when
applied to an interface, this policy can be verified with the show policy-map interface
command. The use of this command is currently limited to the FlexWAN module.
Classification and Marking at Layer 2
Before the days of VoIP and IP-based video conferencing, Layer 3 packet marking was the
only packet marking that was ever discussed. Seemingly, Layer 3 markings were all that
would ever be needed; the reasoning behind that line of thought was that the links in the
switched environment were so fast that there would never be a need to prioritize access to
bandwidth. Of course, VoIP and other real-time traffic also have very low tolerance for
delay or jitter. Therefore, a clear need exists to place bounds on the amount of delay and
jitter that a given packet will experience in the switched network.
A method of marking frames at Layer 2 was developed to allow differentiation of frames
in terms of the characteristics that those frames would receive as they traverse the switched
infrastructure. However, the only way to mark frames at Layer 2 is in the ISL or 802.1Q
header. The location of bits used for this purpose is different for ISL and 802.1Q, as illus-
trated in Figure 2-5.
Figure 2-5 The Layer 2 Class of Service Bits
Mapping Layer 2 to Layer 3 Values
As frames/packets move from the Layer 2 environment to a Layer 3 environment, the ISL
or 802.1Q header is lost. To preserve end-to-end QoS, this loss creates a need for the ability
to map Layer 2 CoS values to Layer 3 ToS values (either IP precedence or DSCP). As
frames/packets move back from the routed network to the switched network, ISL or 802.1Q
headers will again be able to carry Layer 2 QoS markings. This is especially important
when Layer 2 devices that have no Layer 3 capabilities are involved, because they may
understand CoS markings, but not ToS markings. Again, to preserve end-to-end QoS, it
Three Least Significant Bits
of User Field Used for CoS
Layer 2
ISL
ISL Header
26 Bytes
Encapsulated Frame
FCS
4 Bytes
Three Bits Used for CoS
(User Priority bits)
Layer 2
802.1q/p
PTPREAM. SFD DA SA
TAG
4 Bytes
DATA FCS
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