Cisco Systems OL-7396-01 IP QoSFunctional Differences Between Modules

22-11
ATM Switch Router Software Configuration Guide
OL-7396-01
Chapter22 Configuring Quality of Service
IP QoS—Functional Differences Between Modules (Catalyst8540 MSR)
Congestion Control
Two drop policies are supported, tail drop and XPIF based Random Early Detect (or xRed).
Tail Drop
Queues fill up during periods of congestion. When the output queue is full and tail drop is in effect,
packets are dropped until the congestion is eliminated and the queue is no longer full.
On the Catalyst 8540, the point at which packets will start getting dropped is the user configured discard
limit - as soon as the buffer filling drops below this threshold, packets will no lon ger be dropped) This
is the default congestion avoidance mechanism.
xRED
This is a variation of the Random Early Detection Algorithm, as implemented on the Catalyst 8540.
A packet is EFCI-marked if the length of the queue in which it is buffered exceeds a pre-set marking
threshold. By counting the number of EFCI-marked packets over an interval at an output port, the degree
of congestion of the output port can be assessed.
In a given time interval, if Ne represents the total nu mber of EFCI marked packets and Nt represen ts the
total number of packets, then the ratio Ne/Nt follows the average queue length.
Thus, the port's average queue length is monitored, and packets are randomly discarded with a variable
probability if the average queue length exceeds the configured threshold.
Configuring IP QoS Policies Using the Modular CLI
This section describes the tasks for configuring IP QoS functionality with the Modular QoS CLI.
For a complete description of the commands mentioned in this section, refer to the Cisco IOS Quality of
Service Solutions Command Reference. The commands are listed alphabetically within the guide. To
locate documentation of a specific command, use the command reference, master i ndex, or on-line
search.
Note The Catalyst 8500 does not support all the commands docum ented in the Quality of Service Solutions
Command Reference.
IP QoS—Functional Differences Between Modules
(Catalyst 8540 MSR)
This section lists the basic differences in IP QoS functionality for the enhanced Gigabit Ethernet
(XPIF based) interface module and the enhanced ATM Router Module. It also provides an introd uction
to differentiated services for ATM forum VCs and describes their configuration commands.