36-16
Cisco IE 3000 Switch Software Configuration Guide
OL-13018-03
Chapter 36 Configuring QoS
Understanding QoS
Figure 36-8
Queueing and Scheduling Flowchart for Egress Ports
Each port supports four egress queues, one of which (queue 1) can be the egress expedite queue.These
queues are configured by a queue-set. All traffic leaving an egress port flows through one of these four
queues and is subjected to a threshold based on the QoS label assigned to the packet.
Figure 36-9
shows the egress queue buffer. The buffer space is divided between the common pool and
the reserved pool. The switch uses a buffer allocation scheme to reserve a minimum amount of buffers
for each egress queue, to prevent any queue or port from consuming all the buffers and depriving other
queues, and to control whether to grant buffer space to a requesting queue. The switch detects whether
the target queue has not consumed more buffers than its reserved amount (under-limit), whether it has
consumed all of its maximum buffers (over limit), and whether the common pool is empty (no free
90565
Receive p
a
cket from
the intern
a
l ring.
Re
a
d Qo
S
l
a
bel
(D
S
CP or Co
S
v
a
lue).
Determine egre
ss
queue
number
a
nd thre
s
hold
b
as
ed on the l
a
bel.
Are thre
s
hold
s
being exceeded?
S
end the p
a
cket
out the port.
Queue the p
a
cket.
S
ervice
the queue
a
ccording to
the
S
RR weight
s
.
Drop p
a
cket.
S
t
a
rt
Done
Ye
s
No
Rewrite D
S
CP
a
nd/or
Co
S
v
a
lue
as
a
ppropri
a
te.
