QoS

Quality Of Service (QoS)

In the network, there are different kind of traffic running back and forth across the link such as web traffic, voice, video and so on. In the case of congestion, if we treat every traffic as a same manner, it will impact the performance of some application especially real time application such as voice because voice is delay sensitive. In order to ensure performance of the application, QoS, therefore, comes into play.

In an IP packet, there is a 1-byte ToS field which refer to a type of service. ToS contains either IP precedence or DSCP value. These values differentiate traffic. For IP Precedence, there are 8 value (0-7) and DSCP (0-64).

We can assign either IP precedence or DSCP by using a policy-map according to the guide line below.

IP precedence

• 000 (0) - Routine
• 001 (1) - Priority
• 010 (2) - Immediate
• 011 (3) - Flash
• 100 (4) - Flash Override
• 101 (5) - Critical
• 110 (6) - Internetwork Control
• 111 (7) - Network Control

DSCP

Step to assign ToS to packet

• Identify traffic type : we may use access-list or prefix-list to accomplish this. 
• Match access-list by class-map statement.
• User Policy-map to assign type of service to the packet

Example


//Create Access-list
access-list 1 permit 172.16.1.0 0.0.0.255

access-list 101 permit ip 192.168.1.0 0.0.0.255 host 10.1.1.1

//Match access-list to class

!

class-map match-all network-172

 match access-group 1

class-map match-all network-192

 match access-group 101

!

// Apply policy-map to each class including class-default 

policy-map set-qos

 class network-192

  set ip precedence 5

 class network-172

  set ip dscp cs2

 class class-default

  set precedence 0