Input Scheduling 149
between the two versions. The Catalyst 3550 Family of switches is upgradable to the EMI
version. Refer to the product release notes for details.
NOTE In Cisco IOS 12.1(9)EA1, several software changes occurred that optimized the use of the
TCAM space for QoS and security ACLs. As a result, both the Catalyst 2950 and 3550
Family of switches possibly support more ACL entries in Cisco IOS 12.1(9)EA than
previous versions.
Global and Default Configuration
The Catalyst 2950 Family of switches does not require global configuration to enact on QoS
configuration. The default trust behavior of the Catalyst 2950 is untrusted. The Catalyst
2950 Family of switches uses the default port CoS value of zero by default for the internal
DSCP value. See the “Internal DSCP and Mapping Tables” section later in this chapter for
more detailed information about internal DSCP.
The Catalyst 3550 Family of switches does support a global QoS configuration. By default,
the QoS is disabled on the Catalyst 3550 Family of switches and, as a result, does not
modify any CoS or DSCP value of a packet on ingress or egress. In addition, the switch
does not carry out any congestion management or avoidance mechanisms. Use the
following configuration command to globally enable QoS on a Catalyst 3550 switch:
[no] mls qos
Example 6-1 illustrates a user globally enabling QoS and verifying the configuration on a
Catalyst 3550 switch.
The default trust behavior of the Catalyst 3550 Family of switches is untrusted when QoS
is enabled. Furthermore, the switch carries out WRR scheduling with each queue receiving
25 percent of transmit bandwidth by default. The “Congestion Management” section of this
chapter discusses WRR in more detail.
Input Scheduling
As with the other Catalyst platforms, neither the Catalyst 2950 nor the 3550 Family of
switches support input scheduling, and they perform only First-In, First-Out (FIFO)
Example 6-1 Enabling and Viewing QoS Global Configuration on a Catalyst 3550 Switch
Switch#config terminal
Switch(config)#mls qos
Switch(config)#end
Switch#show mls qos
QoS is enabled globally
..................Content has been hidden....................
You can't read the all page of ebook, please click here login for view all page.