MQC QoS - Bandwidth allocation & Congestion.

Answered Question
Jan 19th, 2008

Dear All Experts,

Good Day.

I would like to seek for your expertise to resolve the MQC Bandwidth allocation Question.

Please view the below scenario first. Note= Cisco IOS = 12.4.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~

policy Z

Class A

bandwidth percent 50

Class B

bandwidth percent 30

int serial0/0

bandwidth 256

max-reserved-bandwidth 90

from the above scenarior, we can calculate like that

Max-reserved-bandwidth = 25.6

Bandwidth of Class A = 128

Bandwidth of Class B = 76

Available bandwidth of Serial0/0 = 26

Cisco2600#show int serial0/0

Serial0/3/0 is up, line protocol is up

Hardware is HWIC-Serial

.........

Queueing strategy: Class-based queueing

Output queue: 0/1000/64/0 (size/max total/threshold/drops)

Conversations 0/65/64 (active/max active/max total)

Reserved Conversations 2/2 (allocated/max allocated)

Available Bandwidth 26 kilobits/sec

...........

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

My questions are"

1. What is the available bandwidth 26 kilobits for ? it is for class-default ?

We know that If class-default does not use the bandwidth, the unused bandwidth is available for use by class A and class B. If both classes need the bandwidth, they share it in proportion to the configured rates.

2. Let's say now Class A doest use the allocated bandwidth or not congestion happened, the free bandwidth of class A will give Class B or not ? or it just reserve for own classes used ?

3. Now, Class A already use exceed allocated 50% bandwidth, is it possible to gain some bandwidth from free Classes like Class B ?

Please kindly answer my questions :)

Really appreciate it @@

Regards

Terry

I have this problem too.
0 votes
Correct Answer by Edison Ortiz about 8 years 10 months ago

1) It should display packets being matched on the first entry

___

Class-map: B (match-any)

0 packets, 0 bytes

5 minute offered rate 0 bps, drop rate 0 bps

Match: access-group name Z

0 packets, 0 bytes

5 minute rate 0 bps

____

Then it will increase packets matched on the second entry if there is congestion.

____

Queueing

Output Queue: Conversation 74

Bandwidth 30 (%)

Bandwidth 76 (kbps)

(pkts matched/bytes matched) 0/0

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

exponential weight: 8

mean queue depth: 0

____

2. Classes don't share their guaranteed bandwidth during congestion.

3. As the ACL matches nothing, this allocated bandwidth goes to waste. It's not given to any of the classes and taking away from the total available bandwidth from class-default during congestion. In other words, you are taking 76kbps from class-default during congestion as Class B isn't doing nothing at the moment.

__

Edison.

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Overall Rating: 4 (1 ratings)
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Edison Ortiz Sat, 01/19/2008 - 13:41

1) Yes, it left for remaining classes which in this case is class-default.

2) Yes, if there isn't any congestion, any class can use bandwidth above their reserved value.

3) see response above. Bandwidth only kicks in on congestions, it does not set a limiter like when you use priority.

HTH,

___

Edison.

terry_leong Sat, 01/19/2008 - 23:23

Dear ediortiz,

Good Day,

Really appreciate your fast response.

Need your expertise again for the following scenario,

Please view the attachment first.

From the attachment, we can see that Class A experience congestion and Class B didn't match anything so far.

My Questions are:

1. What kind of condition will cause the congestion ? Is it most of packet came from Class A and Class Default, due to class B match nothing yet ?

2. Let's say the Class A use the bandwidth up 190kbps, and other traffic use the rest bandwidth. Now a network congestion has happened, Class B will still provide free bandwidth to Class A ? or reserve for own use due the network congestion ? If classB will not give the free bandwidth to Class A, the available bandwidth of Class A will drop to 128kbps and random discard the packet (WRED enable).

3. The Class B access-group name Z match nothing, is it no any packet matched the access-list Z ?or the access-group only record match as congestion happened or mark each packet whatever congestion or not? if it is marking each packet whatever congestion or not, is it the access-group designed wrongly ?Or assisgn more bandwidth to Class A as congestion occur.

Please kindly resolve my question :)

Thanks alot.

Regards

Terry

Edison Ortiz Sun, 01/20/2008 - 08:20

Please post access-group name Z and access-group name X.

1) I believe you aren't seeing anything on Class B is due to the incorrect ACL

2) Class A will have 50% of the total bandwidth guaranteed (128kbps). Any amount over 128kbps will be queued as the rest of the traffic. Classes don't share their guaranteed %.

3) I need to see both ACLs. You need to match something to allocate guaranteed bandwidth, otherwise this is not a well designed MQC.

__

Edison.

terry_leong Sun, 01/20/2008 - 09:02

Dear Edison,

Good Morning.

Access-group name Z and access-group name X are just a normal access-list.

1. I didn't see anything match on class B, Yeah, it should be incorrect ACL. Bro, from the command #show policy-map int serial0/0, is it whatever congestion or not, the command will accumulate/record the count of packet and pkts matched ? or only on congestion condition ?

2. For Class A, any amount over 128kbps will be queued as the rest of the traffic. Using the previous scenario, Now congestion has happened Class A Access-group X match the packet up to 190kbps, so the 62kbps will be queued as the rest traffic. Due to Class B match nothing yet, it has free 76kbps and then the rest 62kbps will use the class B bandwidth or class default ? or Classes don't share their guaranteed during congestion.

3. Like the scenerio 2, Class B match nothing so it should not be well designed MQC. If Class B match nothing(ACL wrongly), the bandwidth will reserve or not as congestion happened. same question.

Thank for your expertise.

Regards

Terry

Correct Answer
Edison Ortiz Sun, 01/20/2008 - 10:18

1) It should display packets being matched on the first entry

___

Class-map: B (match-any)

0 packets, 0 bytes

5 minute offered rate 0 bps, drop rate 0 bps

Match: access-group name Z

0 packets, 0 bytes

5 minute rate 0 bps

____

Then it will increase packets matched on the second entry if there is congestion.

____

Queueing

Output Queue: Conversation 74

Bandwidth 30 (%)

Bandwidth 76 (kbps)

(pkts matched/bytes matched) 0/0

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

exponential weight: 8

mean queue depth: 0

____

2. Classes don't share their guaranteed bandwidth during congestion.

3. As the ACL matches nothing, this allocated bandwidth goes to waste. It's not given to any of the classes and taking away from the total available bandwidth from class-default during congestion. In other words, you are taking 76kbps from class-default during congestion as Class B isn't doing nothing at the moment.

__

Edison.

shiva_ial Mon, 01/21/2008 - 04:10

Hi Terry,

i have same doubt on the qos part

but not with bandwidth command (bandwidth-percent-command which cisco going to remove the command on recent ios)

from the post you like to know how qos kicks in

there are two types of queues

software queues

hardware queues

qos is a kind of software queue,kicks in only when hardware queue gets filled(hardware queue

uses only FIFO)software queue is behind the

hardware queue, so when hardware queue gets filled software queue behind it kicks in

post comments if i am confusing

rgds

shiva

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