Chapter 10
| Quality of Service
Creating QoS Policies
– 232 –
Creating QoS Policies
Use the Traffic > DiffServ (Configure Policy) page to create a policy map that can be
attached to multiple interfaces. A policy map is used to group one or more class
map statements (page 228), modify service tagging, and enforce bandwidth
policing. A policy map can then be bound by a service policy to one or more
interfaces (page 241).
Configuring QoS policies requires several steps. A class map must first be
configured which indicates how to match the inbound packets according to an
access list, a DSCP or IP Precedence value, or a member of specific VLAN. A policy
map is then configured which indicates the boundary parameters used for
monitoring inbound traffic, and the action to take for conforming and non-
conforming traffic. A policy map may contain one or more classes based on
previously defined class maps.
The class of service or per-hop behavior (i.e., the priority used for internal queue
processing) can be assigned to matching packets. In addition, the flow rate of
inbound traffic can be monitored and the response to conforming and non-
conforming traffic based by one of three distinct policing methods as described
below.
Police Flow Meter – Defines the committed information rate (maximum
throughput), committed burst size (burst rate), and the action to take for
conforming and non-conforming traffic.
Policing is based on a token bucket, where bucket depth (that is, the maximum
burst before the bucket overflows) is specified by the “burst” field (BC), and the
average rate tokens are removed from the bucket is specified by the “rate” option
(CIR). Action may be taken for traffic conforming to the maximum throughput, or
exceeding the maximum throughput.
srTCM Police Meter – Defines an enforcer for classified traffic based on a single rate
three color meter scheme defined in RFC 2697. This metering policy monitors a
traffic stream and processes its packets according to the committed information
rate (CIR, or maximum throughput), committed burst size (BC, or burst rate), and
excess burst size (BE). Action may taken for traffic conforming to the maximum
throughput, exceeding the maximum throughput, or exceeding the excess burst
size.
◆ The PHB label is composed of five bits, three bits for per-hop behavior, and two
bits for the color scheme used to control queue congestion. In addition to the
actions defined by this command to transmit, remark the DSCP service value, or
drop a packet, the switch will also mark the two color bits used to set the drop
precedence of a packet. A packet is marked green if it doesn't exceed the
committed information rate and committed burst size, yellow if it does exceed
the committed information rate and committed burst size, but not the excess
burst size, and red otherwise.
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