312
CN4093 Application Guide for N/OS 8.4
Priority-Based Flow Control
Priority
‐
based
Flow
Control
(PFC)
is
defined
in
IEEE
802.1Qbb.
PFC
extends
the
IEEE
802.3x
standard
flow
control
mechanism.
Under
standard
flow
control,
when
a
port
becomes
busy,
the
switch
manages
congestion
by
pausing
all
the
traffic
on
the
port,
regardless
of
the
traffic
type.
PFC
provides
more
granular
flow
control,
allowing
the
switch
to
pause
specified
types
of
traffic
on
the
port,
while
other
traffic
on
the
port
continues.
PFC
pauses
traffic
based
on
802.1p
priority
values
in
the
VLAN
tag.
The
administrator
can
assign
different
priority
values
to
different
types
of
traffic
and
then
enable
PFC
for
up
to
two
specific
priority
values:
priority
value
3,
and
one
other.
The
configuration
can
be
applied
on
a
port
‐
by
‐
port
basis,
or
globally
for
all
ports
on
the
switch.
Then,
when
traffic
congestion
occurs
on
a
port
(caused
when
ingress
traffic
exceeds
internal
buffer
thresholds),
only
traffic
with
priority
values
where
PFC
is
enabled
is
paused.
Traffic
with
priority
values
where
PFC
is
disabled
proceeds
without
interruption
but
may
be
subject
to
loss
if
port
ingress
buffers
become
full.
Although
PFC
is
useful
for
a
variety
of
applications,
it
is
required
for
FCoE
implementation
where
storage
(SAN)
and
networking
(LAN)
traffic
are
converged
on
the
same
Ethernet
links.
Typical
LAN
traffic
tolerates
Ethernet
packet
loss
that
can
occur
from
congestion
or
other
factors,
but
SAN
traffic
must
be
lossless
and
requires
flow
control.
For
FCoE,
standard
flow
control
would
pause
both
SAN
and
LAN
traffic
during
congestion.
While
this
approach
would
limit
SAN
traffic
loss,
it
could
degrade
the
performance
of
some
LAN
applications
that
expect
to
handle
congestion
by
dropping
traffic.
PFC
resolves
these
FCoE
flow
control
issues.
Different
types
of
SAN
and
LAN
traffic
can
be
assigned
different
IEEE 802.1p
priority
values.
PFC
can
then
be
enabled
for
priority
values
that
represent
SAN
and
LAN
traffic
that
must
be
paused
during
congestion,
and
disabled
for
priority
values
that
represent
LAN
traffic
that
is
more
loss
‐
tolerant.
PFC
requires
CEE
to
be
turned
on
(
When
CEE
is
turned
on,
PFC
is
enabled
on
priority
value
3
by
default.
Optionally,
the
administrator
can
also
enable
PFC
on
one
other
priority
value,
providing
lossless
handling
for
another
traffic
type,
such
as
for
a
business
‐
critical
LAN
application.
Note:
For
any
given
port,
only
one
flow
control
method
can
be
implemented
at
any
given
time:
either
PFC
or
standard
IEEE
802.3x
flow
control.
Summary of Contents for Flex System Fabric CN4093
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