QOS - MQC and class Based Markings Demystified

The 6-bit DSCP field (described in RFC 2474) defines the per-hop behavior (PHB). A PHB is an externally observable
forwarding behavior or QoS treatment performed by a network device such as a router or a switch.

The four different DiffServ PHBs are Best Effort (BE), Class Selector (CS), Assured Forwarding (AF), and Expedited
Forwarding (EF):

n BE is indicated when all 6 bits of the DS field are zero, and it has no specific QoS treatment.
n CS is used for backward compatibility with IP Precedence, and when using this PHB, the last 3 bits of the DSCP
field are zero.
n AF (defined in RFC 2597) specifies four different classes, along with three different drop precedences.
When using AF, the first 3 bits of the DS field define the queuing class (1 to 4), and the last 3 bits define the drop
precedence (the likelihood of the packet being dropped [1 to 3]). AF PHB names are often written in the AFxy
format, where x is the queuing class and y is the drop precedence.
n EF (RFC 3246) specifies a low delay, low jitter, and low packet-loss QoS treatment with a bandwidth guarantee.


Follow the sequence of commands mentioned above to configure class based marking of trafficStep 1 The following commands need to be entered on R1 router.

access-list 101 permit tcp any any eq ftp
access-list 101 permit tcp any any eq ftp-data

Configuration 1: Configuration description

Step 2 The following commands need to be entered on R1 router.
access-list 102 permit tcp any any eq www

Configuration 2: Configuration description


Step 3 The following commands need to be entered on R1 router.
class-map match-ftp
match access-group 101
!
class-map match-www
match access-group 102

Configuration 3:
Configuration description
Step 4 The following commands need to be entered on R1 router.
policy-map mark-apps
class match-ftp
set dscp af11

class match-www
set dscp default

Configuration 4: Configuration description

Step 5 The following commands need to be entered on R1 router.
interface fastethernet0/0
service-policy input mark-apps

Configuration 5: Configuration description

Step 6 Class FTP: matched_ 1749__ marked_ 1749__
Class WWW: matched_ 1028__ marked_ 1028__
Class class-default: matched_ 28733_