QoS Newbie Questions

[asaykao73]

Hi All,

I'm fairly new to the wonderful world of QoS and have a few basic questions to ask.

1/ For physical interfaces that are greater than the actual WAN speed, do we always need to set a parent shaper to shape/police the interface for QoS to recognize what the "real" bandwidth is.

Eg:

[pop1:GigE] <-- 250mb --> [pop2:GigE]

Here I have a WAN link of 250mb between two pops, of which both are using a GigE interface. If I apply my QoS policy, say at pop2, do I need to set up a parent shaper (hierarchical qos) for QoS to use the actual WAN speed rather than the GigE interface speed.

class-map match-all POP2-CLASS

match access-group POP2-ACL

!

policy-map POP2-CHILD

class POP2-CLASS

bandwidth percent 5

class class-default

random-detect

policy-map POP2-PARENT

class class-default

shape average 200000000

service-policy POP2-CHILD

!

interface GigabitEthernet4/0/2

bandwidth 200000

service-policy output POP2-PARENT

2/ How do I determine if the QoS policy is working? Everyone keeps saying that QoS will only come into play when there's congestion on the line.

For my example above, if the WAN link is only pushing 120mb (max is 250mb), am I expecting to see matches when I do a "show policy-map interface" eventhough there is no congestion???

POP2#sh int g4/0/2

30 second input rate 49731000 bits/sec, 18293 packets/sec

30 second output rate 117390000 bits/sec, 20314 packets/sec

POP2#sh policy-map int g4/0/2

GigabitEthernet4/0/2

Service-policy output: POP2-PARENT

Counters last updated 00:00:00 ago

Class-map: class-default (match-any)

2380971 packets, 1747294017 bytes

30 second offered rate 114932000 bps, drop rate 0 bps

Match: any

Queueing

queue limit 62500 packets

(queue depth/total drops/no-buffer drops) 0/0/0

(pkts output/bytes output) 2305103/1694206052

shape (average) cir 250000000, bc 1000000, be 1000000

target shape rate 250000000

Service-policy : POP2-CHILD

Counters last updated 00:00:00 ago

Class-map: POP2-CLASS (match-all)

47356 packets, 17678197 bytes

30 second offered rate 1196000 bps, drop rate 0 bps

Match: access-group name POP2-ACL

Queueing

queue limit 3125 packets

(queue depth/total drops/no-buffer drops) 0/0/0

(pkts output/bytes output) 45925/17159720

bandwidth 5% (12500 kbps)

Class-map: class-default (match-any)

2333619 packets, 1729616064 bytes

30 second offered rate 113738000 bps, drop rate 0 bps

Match: any

queue limit 59375 packets

(queue depth/total drops/no-buffer drops) 0/0/0

(pkts output/bytes output) 2259178/1677046332

Exp-weight-constant: 9 (1/512)

Mean queue depth: 4 packets

If there's no congestion at the moment, why are the classes being offered an "offered rate"?

Thanks in advance.

Andy

[rpfinneran]

1. No, ensure your "bandwidth" commands are set correctly on all interfaces. QoS will use this value.

2. Everyone is lying to you :). Yes, you should see matches and indeed you do.

[Joseph W. Doherty]

#1 Need you always shape? No. Should you sometimes shape? Yes.

It's not question of knowing the "real" bandwidth, but it's a question of QoS control. If you don't shape, congestion often forms first, or mostly, at the slower bandwidth handoff point. If the downstream congestion point manages traffic as we desire, there's no need to shape it, and there's reason not too (since a shaper doesn't behave exactly as a physical interface). If the downstream point doesn't manage traffic as we desire, shaping allows us to emulate and manage the downstream congestion before that physical point (which should preclude any/most congestion at that downstream point).

#2 "How do I determine if the QoS policy is working?"

Often by the stats.

"Everyone keeps saying that QoS will only come into play when there's congestion on the line. "

Depends what is meant by "come into play". It might be a bit more correct to say if there's never congestion, which is often very, very uncommon, there's never a need for QoS.

"For my example above, if the WAN link is only pushing 120mb (max is 250mb), am I expecting to see matches when I do a "show policy-map interface" eventhough there is no congestion???"

But you are seeing matches, which is related to your last question "If there's no congestion at the moment, why are the classes being offered an "offered rate"? ". The "offered rate" is the volume of traffic being presented to the different policy map classes.

Oh, and although your stats don't show any congestion at the moment, they indicate quite a bit of some on-going congestion.