Cisco Systems Determining the Queue and TX Ring Limits for a Standard Priority Queue

23-7
Cisco ASA Series Firewall ASDM Configuration Guide
Chapter23 Configuring QoS
Configuring QoS
Determining the Queue and TX Ring Limits for a Standard Priority Queue
To determine the priority queue and TX ring limits, use the worksheets below.
Table23-1 shows how to calculate the priority queue size. Because queues are not of infinite size, they
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 “Configuring the Standard Priority Queue for an Interface” section on page 23-8.
Table23-2 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.
Table23-1 Queue Limit Worksheet
Step1
__________
Outbound
bandwidth
(Mbps or Kbps)1
1. For example, DSL might have an uplink speed of 768 Kbps. Check with your provider.
Mbps x125 =__________
# of bytes/ms
Kbps x.125 =__________
# of bytes/ms
Step2
___________
# of bytes/ms
from Step1
÷
__________
Average packet
size (bytes)2
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.
x__________
Delay (ms)3
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.
=__________
Queue limit
(# of packets)
Table23-2 TX Ring Limit Worksheet
Step1
__________
Outbound
bandwidth
(Mbps or Kbps)1
1. For example, DSL might have an uplink speed of 768 Kbps.Check with your provider.
Mbps x125 =__________
# of bytes/ms
Kbps x0.125 =__________
# of bytes/ms
Step2
___________
# of bytes/ms
from Step1
÷
__________
Maximum packet
size (bytes)2
x__________
Delay (ms)3
=__________
TX ring limit
(# of packets)