QoS - Packet Classification

[Ben_7]

Hi Guys,

I have the following configuration on my WAN interface. It is a 20 Mbs link.

Since the interface is set at 100Mbs, and at 20 %, the bandwidth for COS3 is 20Mbs. I wanted to know how bandwidth are allocated for the default class. 

When COS3 traffic is less than 20%, should class default traffic should have no problem taking into account that its bandwidth is the size of the link? 
but when COS3 is at max utilization, will class default traffic be drop at the policer or the interface?

policy-map qos-policing
class COS3-policing
bandwidth percent 20
class class-default
random-detect dscp-based
fair-queue
service-policy output qos-policing

========================================

Class-map: COS3-policing (match-any)
0 packets, 0 bytes
30 second offered rate 0000 bps, drop rate 0000 bps
Match: ip dscp af11 (10)
0 packets, 0 bytes
30 second rate 0000 bps
Queueing
queue limit 64 packets
(queue depth/total drops/no-buffer drops) 0/0/0
(pkts output/bytes output) 0/0
bandwidth 20% (20000 kbps)

=================================

Cheers,
Ben

[mlund]

Hi Ben

I'm not sure I fully understand You.

You ask about a policer, but there are no policer configured.

If You mean the provider policer, and Your config is what You have show. Then there will be problem.

Your router thinks it can run 100M, and the bandwidth reservation of 20% will only kick in if the interface will be fully loaded (100M). So in that case the provider will randomly drop traffic that is over the 20M.

If You should be sure, You have to configure nested QoS. Where you have Your own shaping to 20M.

/Mikael

[Ben_7]

Hi Mikael,

Thanks for the input.

Yes i meant on the provider's side - sorry for the misunderstanding.

Also about when COS3 will be a max utilization (i.e 20Mbps), will all traffic at class default be drop on my policy? 

Rgrds,
Ben.

[Joseph W. Doherty]

How much bandwidth your class default obtains, depends on whether your CBWFQ is pre or post HQF IOS variant, especially when using FQ. For HQF, I believe it also depends on whether a bandwidth allocation has been set for class default.

BTW, Mikael has touched on this, but you might misunderstand how the bandwidth statement works in a CBWFQ policy or how such a policy works when the physical interface offers more bandwidth than is actually available.

[Ben_7]

Hi Joseph,

Well my confusion is rather on the bandwidth statement based on the interface bandwidth, and how traffic will be allocated to default class if COS 3 traffic is at 20 Mb. Considering that the line is only 20 Mb.

Thanks,
Ben

[Joseph W. Doherty]

How bandwidth is allocated to the default class depends on whether you're dealing with pre or post HQF and also with whether FQ is being used. Generally, class bandwidth is set as a ratio, so if you only had two (or only two active) classes, one with 20% and the other 40%, they would divide the bandwidth 1:2.

Again, FQ is very different between pre and post HQF. In pre HQF, the class flows compete with other classes' flow and actually WFQ is used, which also takes into account IPPrec. With post HQF, all flows get an equal share of the class aggregate bandwidth.