Information Technology Reference
In-Depth Information
The primary tool used on Cisco routers and switches for classification and marking is the Modular QoS CLI (MQC). Using the
MQC, a class map can be used to classify traffic; a policy map can be used to specify QoS policy, such as marking, queuing, shaping,
policing, and so on; and the QoS policy can then be applied to an interface or VC.
The order of class maps in a configuration file does not matter, but the order in which class maps are referenced within a policy map
does. As with an access list, as soon as a match has been found via a referenced class map, other referenced class maps are ignored. If
no explicitly configured class maps match the traffic in question, this traffic is matched by an implicit default class (class default).
The following example shows configuration of classification and marking using the MQC:
class-map
voice-traffic
match ip rtp 16384 16383
!
policy-map
mark-voice
class
voice-traffic
set dscp ef
!
interface FastEthernet 0/0
ip address
192.168.1.1 255.255.255.0
service-policy input
mark-voice
The class map (
class map
[
match-all
|
match-any
]
class-map-name
) called voice-traffic matches voice media traffic (
match ip rtp
16384 16383
). The policy map called mark-voice references the class map called voice-traffic (
class
{
class-name
|
class-default
})
and sets the DSCP field for traffic that matches that class map to EF (
set dscp
dscp-value
). Finally, policy map mark-voice is applied
to interface serial 0/0 in an input direction using the
service-policy
{
input
|
output
}
policy-map-name
command.
The
match
command can be used to match various frame or packet field settings, including DSCP values, IP Precedence values, CoS
values, MPLS Experimental (EXP) bit values, MAC addresses, criteria specified in an access list, and so on.
The
set
command can be used to modify the settings of the DSCP field, IP Precedence bits, 802.1p/ISL CoS bits, MPLS EXP bits,
Frame Relay Discard Eligibility (DE) bit, ATM Cell Loss Priority (CLP) bit, and so on.
The Frame Relay DE bit is used to indicate to Frame Relay switches which frames should be dropped first in the event of congestion.
If the DE bit is set in a frame, this indicates that the frame should be dropped first (rather than a frame whose DE bit is not set). The
ATM Cell Loss Priority (CLP) bit operates in a similar way in ATM networks.
