How Can I Implement Quality of Service?
February 22, 2008
Page 2 of 29
required,
packet
loss,
or
availability
requirements
of
the
flow.
QoS
provides
management
mechanisms
for
these
flow
characteristics.
QoS
achieves
its
bandwidth
management
capabilities
by:
•
Setting
priorities
that
define
traffic
handling
•
Dedicating
bandwidth
and
prioritizing
queuing
for
specific
applications,
and
reducing
packet
transmission
delay
and
jitter
•
Managing
congestion
by
shifting
packet
loss
to
applications
that
can
tolerate
it
How Can I Implement Quality of Service?
QoS
determines
how
a
flow
will
be
treated
as
it
transits
the
link.
To
determine
how
a
flow
should
be
treated,
you
must
first
understand
the
characteristics
of
the
flows
on
your
network,
and
secondly,
you
must
identify
these
flows
in
a
way
that
QoS
can
recognize.
In
this
sense,
QoS
is
the
third
step
in
a
three
step
process.
The
three
‐
steps
Enterasys
recommends
for
configuring
QoS
are:
•
Understand
your
network
flows
using
NetFlow
•
Associate
the
flows
on
your
network
with
a
well
defined
role
using
Enterasys
policy
•
Configure
the
appropriate
link
behavior
for
that
role
by
associating
the
role
with
a
QoS
configuration
Quality of Service Overview
QoS
is
all
about
managing
the
bandwidth
in
a
manner
that
aligns
the
delivery
characteristics
of
a
given
flow
with
the
available
port
resources.
In
a
QoS
context,
a
flow
is
a
stream
of
IP
packets
that
are
classified
with
the
same
class
of
service
as
it
transits
the
interface.
QoS
manages
bandwidth
for
each
flow
by
taking
advantage
of
its
ability
to:
•
Assign
different
priority
levels
to
different
packet
flows
•
Mark
or
re
‐
mark
the
packet
priority
at
port
ingress
with
a
Type
of
Service
•
Sort
flows
by
transit
queue
such
that
a
higher
priority
queue
gets
preferential
access
to
bandwidth
during
packet
forwarding
•
Limit
the
amount
of
bandwidth
available
to
a
given
flow
by
either
dropping
(rate
limit)
or
buffering
(rate
shape)
packets
in
excess
of
configured
limits
These
QoS
abilities
collectively
make
up
a
Class
of
Service
(CoS).
The
remainder
of
this
section
will
briefly
describe
CoS
and
its
components.
Class of Service (CoS)
You
implement
QoS
features
in
a
Class
of
Service
(CoS).
How
the
firmware
treats
a
packet
as
it
transits
the
link
depends
upon
the
priority
and
forwarding
treatments
configured
in
the
CoS.
Up
to
256
unique
CoS
entries
can
be
configured.
CoS
entries
0–7
are
configured
by
default
with
an
802.1p
priority
assigned
and
default
forwarding
treatment.
For
purposes
of
backward
compatibility,
CoS
entries
0–7
cannot
be
removed.
CoS
entries
8
‐
255
can
be
configured
for
the
following
services:
•
802.1p
priority
•
IP
ToS
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