Chapter 10
| Quality of Service
Creating QoS Policies
– 210 –
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 206). A policy map can then be bound by a service policy to
one or more interfaces (page 214).
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 a specific VLAN. A policy
map is then configured which indicates the boundary parameters used for
monitoring inbound 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.
Meter Mode – Defines the committed information rate (maximum throughput).
◆
Policing is based on a token bucket, where bucket depth is the maximum burst
before the bucket overflows, and the average rate tokens that are added to the
bucket is by specified by the
committed-rate
option. Note that the token bucket
functions similar to that described in RFC 2697 and RFC 2698.
◆
The behavior of the meter is specified in terms of its mode and two token
buckets, C and E, which both share the common rate CIR. The maximum size of
the token bucket C is BC and the maximum size of the token bucket E is BE.
The token buckets C and E are initially full, that is, the token count Tc(0) = BC
and the token count Te(0) = BE. Thereafter, the token counts Tc and Te are
updated CIR times per second as follows:
■
If Tc is less than BC, Tc is incremented by one, else
■
if Te is less then BE, Te is incremented by one, else
■
neither Tc nor Te is incremented.
When a packet of size B bytes arrives at time t, the following happens if srTCM is
configured to operate in Color-Blind mode:
■
If Tc(t)-B ≥ 0, the packet is green and Tc is decremented by B down to the
minimum value of 0, else
■
if Te(t)-B ≥ 0, the packets is yellow and Te is decremented by B down to the
minimum value of 0,
■
else the packet is red and neither Tc nor Te is decremented.
When a packet of size B bytes arrives at time t, the following happens if srTCM is
configured to operate in Color-Aware mode:
■
If the packet has been precolored as green and Tc(t)-B ≥ 0, the packet is
green and Tc is decremented by B down to the minimum value of 0, else
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