Cisco ASA 5505 User Manual

Page 1151

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54-7

Cisco ASA 5500 Series Configuration Guide using the CLI

Chapter 54 Configuring QoS

Configuring QoS

Table 54-2

shows how to calculate the TX ring limit. This limit determines the maximum number of

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.

Configuring the Standard Priority Queue for an Interface

If you enable standard priority queuing for traffic on a physical interface, then you need to also create
the priority queue on each interface. Each physical interface uses two queues: one for priority traffic,
and the other for all other traffic. For the other traffic, you can optionally configure policing.

Note

The standard priority queue is not required for hierarchical priority queuing with traffic shaping; see the

“Information About Priority Queuing” section on page 54-3

for more information.

Restrictions

(ASASM) The ASASM does not support priority queuing.

(ASA 5580) You cannot create a standard priority queue for a Ten Gigabit Ethernet interface. Note:
For the ASA 5585-X, standard priority queuing is supported on a Ten Gigabit Ethernet interface.

(ASA 5512-X through ASA 5555-X) Priority queuing is not supported on the Management 0/0
interface.

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 54-2

TX Ring Limit Worksheet

Step 1

__________

Outbound

bandwidth

(Mbps or Kbps)

1

1.

For example, DSL might have an uplink speed of 768 Kbps.Check with your provider.

Mbps

x

125

=

__________

# of bytes/ms

Kbps

x

0.125

=

__________

# of bytes/ms

Step 2

___________

# of bytes/ms

from Step 1

÷

__________

Maximum packet

size (bytes)

2

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.

x

__________

Delay (ms)

3

3.

The delay depends on your application. For example, to control jitter for VoIP, you should use 20 ms.

=

__________

TX ring limit

(# of packets)

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