Cisco ASA 5505 Configuration Manual page 1064

Configuring QoS
Determining the Queue and TX Ring Limits
To determine the priority queue and TX ring limits, use the worksheets below.
Table 49-1
can fill and overflow. When a queue is full, any additional packets cannot get into the queue and are
dropped (called tail drop). To avoid having the queue fill up, you can adjust the queue buffer size
according to the
Table 49-1
Step 1
__________
Outbound
bandwidth
(Mbps or Kbps)
Step 2
___________
# of bytes/ms
from Step 1
1. For example, DSL might have an uplink speed of 768 Kbps. Check with your provider.
2. Determine this value from a codec or sampling size. For example, for VoIP over VPN, you might use 160 bytes. We recommend 256
bytes if you do not know what size to use.
3. The delay depends on your application. For example, the recommended maximum delay for VoIP is 200 ms. We recommend 500 ms
if you do not know what delay to use.
Table 49-2
packets allowed into the Ethernet transmit driver before the driver pushes back to the queues on the
interface to let them buffer packets until the congestion clears. This setting guarantees that the
hardware-based transmit ring imposes a limited amount of extra latency for a high-priority packet.
Table 49-2
Step 1
__________
Outbound
bandwidth
(Mbps or Kbps)
Step 2
___________
# of bytes/ms
from Step 1
1. For example, DSL might have an uplink speed of 768 Kbps.Check with your provider.
2. Typically, the maximum size is 1538 bytes, or 1542 bytes for tagged Ethernet. If you allow jumbo frames (if supported for your
platform), then the packet size might be larger.
3. The delay depends on your application. For example, to control jitter for VoIP, you should use 20 ms.
Cisco ASA 5500 Series Configuration Guide using ASDM
49-6
shows how to calculate the priority queue size. Because queues are not of infinite size, they
"Configuring the Priority Queue" section on page
Queue Limit Worksheet
x
Mbps 125
1
x
Kbps .125
÷
__________
Average packet
size (bytes)
shows how to calculate the TX ring limit. This limit determines the maximum number of
TX Ring Limit Worksheet
x
Mbps 125
1
x
Kbps 0.125
÷
__________
Maximum packet
size (bytes)
49-7.
=
__________
# of bytes/ms
=
__________
# of bytes/ms
x
__________
3
Delay (ms)
2
=
__________
# of bytes/ms
=
__________
# of bytes/ms
x
__________
3
Delay (ms)
2
Chapter 49 Configuring QoS
=
__________
Queue limit
(# of packets)
=
__________
TX ring limit
(# of packets)
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