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Available bandwindth and 'max-reserved bandwidth'

Unanswered Question
Jul 27th, 2006
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Is the max-reserved bandwidth only important when working with Qos classes and the bandwidth statement? Is the default 75% available bandwidth only used then?

In other words if I have a 100MB link with a service policy applied for Voice, Call-Control and video. After that I notice the available bandwidth on thie 100MB link is 61280 kilobits/sec.

If I put in a 'max-reserved bandwidth 95' would I reclaim another 20MB of bandwidth for the class-default? Would leaving 5% on the 100MB link for routing and other stuff be acceptable?

Here is the config and show commands:

class-map match-any Call-Control

match ip dscp cs3

match ip dscp af31

class-map match-any Video

match ip dscp af41

class-map match-any Voice

match ip dscp ef

policy-map QOS_classes_to_ACN

class Voice

priority 10000

class Call-Control

bandwidth 500

class Video

bandwidth 3220

class class-default



interface FastEthernet6/0

description 100MB Link to ACN

ip address xxx.xxx.xxx.xxx xxx.xxx.xxx.xxx

ip route-cache flow

no ip mroute-cache

load-interval 30

duplex full

speed 100

service-policy output QOS_classes_to_ACN

ROC-RT7206-QMOE#sh int f6/0

FastEthernet6/0 is up, line protocol is up

Hardware is i82543 (Livengood), address is 00b0.4a28.3ca8 (bia 00b0.4a28.3ca8)

Description: 100MB Link to ACN

Internet address is xxx.xxx.xxx.xxx/xx

MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec,

reliability 255/255, txload 183/255, rxload 21/255

Encapsulation ARPA, loopback not set

Keepalive set (10 sec)

Full-duplex, 100Mb/s, 100BaseTX/FX

ARP type: ARPA, ARP Timeout 04:00:00

Last input 00:00:03, output 00:00:00, output hang never

Last clearing of "show interface" counters 01:13:30

Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 5211742

Queueing strategy: Class-based queueing

Output queue: 70/1000/64/5211742 (size/max total/threshold/drops)

Conversations 2/35/256 (active/max active/max total)

Reserved Conversations 2/2 (allocated/max allocated)

Available Bandwidth 61280 kilobits/sec <--- Available bandwidth

30 second input rate 8615000 bits/sec, 6860 packets/sec

30 second output rate 71788000 bits/sec, 7484 packets/sec

31692173 packets input, 4263195179 bytes

Received 1204 broadcasts, 0 runts, 0 giants, 0 throttles

0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored

0 watchdog

0 input packets with dribble condition detected

34536300 packets output, 2513155446 bytes, 0 underruns

0 output errors, 0 collisions, 0 interface resets

0 babbles, 0 late collision, 0 deferred

0 lost carrier, 0 no carrier

0 output buffer failures, 0 output buffers swapped out

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jkeeffe Thu, 07/27/2006 - 09:02
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Here is the output of show policy-pam int:

ROC-RT7206-QMOE#sh policy-map int f6/0


Service-policy output: QOS_classes_to_ACN

Class-map: Voice (match-any)

3417571 packets, 934178998 bytes

30 second offered rate 1722000 bps, drop rate 0 bps

Match: ip dscp ef (46)

3417571 packets, 934178998 bytes

30 second rate 1722000 bps


Strict Priority

Output Queue: Conversation 264

Bandwidth 10000 (kbps) Burst 250000 (Bytes)

(pkts matched/bytes matched) 1908656/521903140

(total drops/bytes drops) 0/0

Class-map: Call-Control (match-any)

615085 packets, 48926098 bytes

30 second offered rate 84000 bps, drop rate 0 bps

Match: ip dscp cs3 (24)

588857 packets, 47299978 bytes

30 second rate 81000 bps

Match: ip dscp af31 (26)

26228 packets, 1626120 bytes

30 second rate 2000 bps


Output Queue: Conversation 265

Bandwidth 500 (kbps) Max Threshold 64 (packets)

(pkts matched/bytes matched) 337953/26882724

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

Class-map: Video (match-any)

146136 packets, 82165408 bytes

30 second offered rate 90000 bps, drop rate 0 bps

Match: ip dscp af41 (34)

146136 packets, 82165408 bytes

30 second rate 90000 bps


Output Queue: Conversation 266

Bandwidth 3220 (kbps) Max Threshold 64 (packets)

(pkts matched/bytes matched) 81687/45950190

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

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

35227089 packets, 47492000208 bytes

30 second offered rate 87718000 bps, drop rate 14714000 bps

Match: any


Flow Based Fair Queueing

Maximum Number of Hashed Queues 256

(total queued/total drops/no-buffer drops) 0/5171786/0

exponential weight: 9

class Transmitted Random drop Tail drop Minimum Maximum Mark

pkts/bytes pkts/bytes pkts/bytes thresh thresh prob

0 30181523/39910255774 1297726/1944176143 3893194/5836883998 20 40 1/10

1 0/0 0/0 0/0 22 40 1/10

2 0/0 0/0 0/0 24 40 1/10

3 0/0 0/0 0/0 26 40 1/10

4 0/0 0/0 0/0 28 40 1/10

5 0/0 0/0 0/0 30 40 1/10

6 1213/88749 0/0 0/0 32 40 1/10

7 0/0 0/0 0/0 34 40 1/10

rsvp 0/0 0/0 0/0 36 40 1/10

jackyoung Thu, 07/27/2006 - 18:37
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In a simple ans., Yes, increase the max-reserve bandwidth then you can use more bandwidth. How much you allocate the bandwidth is quite difficult to decide, it depends on how much traffic will be the control, routing protocol, etc.

If your network is simple and not much routes & route update there, I believe you can increase the ratio, 75% is only a default value and you can fine tune it.

So, in this case, if you decided the ratio then you have to recalculate the bandwidth allocation in each class to reflect to the correct throughput.

Hope this helps.

mheusinger Fri, 07/28/2006 - 01:32
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just for clarification (maybe unnecessary, but ...): you always have 100 MB used bandwidth. Queueing, which uses max-reserved and bandwidth to calculate "bandwidth ratios" between different classes, only kicks in once your link is overloaded. Unless this is the case any traffic will be forwarded with line speed.

So max-res and bandwidth (interface command) only modify your per-class bandwidth guarantees in an overload situation.

Hope this helps!

Regards, Martin

royalblues Fri, 07/28/2006 - 02:32
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I had a similar post sometime back but there was no QOS policy applied.

With default queuing too i find the available bandwidth is less than the bandwidth on the link.

And if i just change the bandwidth command to say some arbitrary value on the interface (although it will not increase my line bandwidth), these values change..

So if i have a T1 line and no policies, what would ideally be the available bandwidth?

Thanks in advance


jkeeffe Fri, 07/28/2006 - 07:43
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So if I understand you correctly traffic in the class-default will have full line rate available to them unless there is congestion?

The reason I ask is on this link, along with the classes for voice, video and call control, we are now transferring one huge volume of traffic for data backup. This is a one time operation and it is over a terrabyte. The data transfer folks say they've throttled the application that is transfering this data to 75mbs, which is 3/4 of the bandwidth on this 100mb link. My confusion comes from the 'available bandwidth' that is shown in the 'show int' which is around 61mb. With only 61mb available, and the transfer application is sending at 75mbps there is massive congestions as can be seen by all the dropped packets on the 'show int'.

I thought that if I used the 'max-reserved bandwidth 95' I would gain up to 20mb of throughput. But I guess this isn't the case? Is it true then that increasing the max-reservered bandwidth won't free up more bandwidth for class-default traffic?


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