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Cisco ME 3800X and 3600X Switch Software Configuration Guide
OL-23400-01
Chapter 27 Configuring QoS
Understanding QoS
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VLAN level—You configure per-VLAN QoS by entering the match vlan vlan-id or match
vlan-inner vlan-id class-map configuration command for one or more VLANs.
At the VLAN level, you can:
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Configure the VLANs to police by using the police cir or police cir percent policy command.
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Configure unconditional marking by using the set command.
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Configure the queue to share the available port bandwidth and enable CBWFQ by using the
bandwidth command.
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Configure traffic shaping by using the shape command.
You can also associate a previously defined child policy at the class level with a new service policy
by using the service-policy policy-map class configuration command to apply the class-level policy
only to traffic that matches the VLAN class.
You cannot mix VLAN-level and class-level matches
within a class map.
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Physical level—You can shape or police only the class-default class at the physical level of the
hierarchy by using the shape, police cir, or police cir percent policy-map class configuration
command. Within a policy map, the class-default applies to all traffic that is not explicitly matched
within the policy map but that does match the parent policy. If no parent policy is configured, the
parent policy represents the physical port.
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Configure unconditional marking by using the set command.
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In a physical-level policy map, class-default is the only class that you can configure. You use
the service-policy {input | output} policy-map-name interface configuration command to
attach a hierarchical policy to a physical port or to an EFP.
Classification
Classification distinguishes one kind of traffic from another by examining the fields in the packet header.
When a packet is received, the switch examines the header and identifies all key packet fields. A packet
can be classified based on an ACL, on the DSCP, the CoS, IP precedence, or MPLS EXP value in the
packet, or by the VLAN ID.
Figure 27-2
has examples of classification information carried in a Layer 2
or a Layer 3 IP packet header, using six bits from the deprecated IP type of service (ToS) field to carry
the classification information.
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Layer 2 frame headers have a 2-byte Tag Control Information field that carries the CoS value, called
the User Priority bits, in the 3 most-significant bits, and the VLAN ID value in the 12
least-significant bits. Layer 2 CoS values range from 0 to 7.
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Layer 3 IP packets can carry either an IP precedence value or a DSCP value. QoS supports the use
of either value because DSCP values are backward-compatible with IP precedence values. IP
precedence values range from 0 to 7. DSCP values range from 0 to 63. MPLS EXP values range from
0 to 7.
