Sunday, June 23, 2013

QoS Groups

QoS Groups are locally significant groups that can categorize different classes of traffic together under one group, you can configure 1024 qos groups (0-1023). Since it is only locally significant to the router, it can only be set in the Ingress direction and matched in the Egress direction.

The advantage of the qos-groups command is that it can group different class-maps that has different matches in the ingress direction and give them the same treatment while exiting the router from another interface.

Let's see how it works on this simple topology:

R2 --- R3 --- R1

Now R3 has two class-maps that matches Telnet and ICMP traffic coming from R2 heading to R1 and a policy map that attaches these two class-maps to a single qos-group

Ingress Direction from R2
R3#
class-map match-all telnet
 match protocol telnet
class-map match-all icmp
 match protocol icmp
policy-map input
 class telnet
  set qos-group 1
 class icmp
  set qos-group 1
interface FastEthernet1/0
 ip address 10.2.3.3 255.255.255.0
 ip ospf 1 area 0
 duplex auto
 speed auto
 !
 service-policy input input
Now let's check the Egress policy to R1

Egress Direction to R3
R3#
class-map match-all output
 match qos-group 1
policy-map output
 class output
  set precedence 5
interface FastEthernet1/1
 ip address 10.1.3.3 255.255.255.0
 ip ospf 1 area 0
 duplex auto
 speed auto
 !
 service-policy output output
end
As shown in the example above, both class-maps will be treated the same as they will be marked with precedence 5 while exiting the interface towards R1.

Of course the class-map can be more complicated than that.. it may include all the conventional matches of MQC but this is only for illustration.