QOS - using Max-Reserved-Bandwidth command

Answered Question
Jul 16th, 2008

Hi, I'm interested in how folks implement max-reserved-bandwidth with their qos(strict priority/CBWFQ/etc.)

any help is appreciated

I have this problem too.
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Correct Answer by JosephDoherty about 5 years 9 months ago

Yes, default class should be working.

On many Cisco routers, bandwidth in the default class configures a FIFO queue. Without it, normal default is flow based fair-queue. Also on most routers, default class flow queues compete for bandwidth with defined classes. However, the LLQ, when there's congestion, preempts all the other queues (up to its defined cap).

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mounir.mohamed Wed, 07/16/2008 - 09:03

Hi,

The sum of all bandwidth allocation on an interface should not exceed 75 percent of the available bandwidth on an interface. The remaining 25 percent of bandwidth is used for overhead, including Layer 2 overhead, control traffic, and best-effort traffic, so If you need to allocate more than 75 percent for CBWFQ and IP RTP Priority, you can use the this command,

here is an example, below we have 3 different classes (HTTP/FTP/ICMP)in addition to the default class, i will allocate 500k for each class and only 44k for the default class,

since CBWFQ can not allocate more than 75% of the interface bandwidth the above bandwidth allocation doesn't comply with default bandwidth of serial interfaces which is 1544 (1544x25/100=386 so 1544-386=1158) so 1158 is the maximum available bandwidth for allocation on this interface, so the router will never accept policy-map with bandwidth over than 1158 unless we use max-reserved-bandwidth command.

PE1#conf t

PE1(config)#class-map http

PE1(config-cmap)#match protocol http

PE1(config-cmap)#class-map ftp

PE1(config-cmap)#match protocol ftp

PE1(config-cmap)#class-map icmp

PE1(config-cmap)#match protocol icmp

PE1(config)#policy-map qos

PE1(config-pmap)#class http

PE1(config-pmap-c)#bandwidth 500

PE1(config-pmap-c)#class ftp

PE1(config-pmap-c)#bandwidth 500

PE1(config-pmap-c)#class icmp

PE1(config-pmap-c)#bandwidth 500

PE1(config-pmap-c)#class class-default

PE1(config-pmap-c)#bandwidth 44

PE1(config-pmap)#int s2/1

PE1(config-if)#service-policy output qos

I/f Serial2/1 class icmp requested bandwidth 500 (kbps), available only 158 (kbps)

PE1#show queueing int s2/1 | inc Bandwidth

Available Bandwidth 1158 kilobits/sec

PE1(config-if)#max-reserved-bandwidth 100

PE1(config-if)#do show queueing int s2/1 | inc Bandwidth

Available Bandwidth 1544 kilobits/sec

PE1(config-if)#service-policy output qos

PE1(config-if)#do show queueing int s2/1 | inc Bandwidth

Available Bandwidth 0 kilobits/sec

Best Regards,

Mounir Mohamed

rll3@visto.com Wed, 07/16/2008 - 09:25

Thanks for the the information. My concern is with how I need to implement the command. I have a config similar to :

Building configuration...

Current configuration : 281 bytes

!

interface GigabitEthernet0/1

description WLAN VLAN 10.226.70.x/24 (10.226.70.4)

ip address 10.26.7.1 255.255.255.0

duplex auto

speed auto

no snmp trap link-status

max-reserved-bandwidth 95

service-policy output VOICE-WAN

##########sh class-map

Class Map match-any class-default (id 0)

Match any

Class Map match-any VOICE (id 1)

Match ip dscp ef (46)

Match ip dscp cs3 (24)

Match ip dscp af31 (26)

Match protocol rtp audio

Class Map match-all KEHTrfc (id 2)

Match access-group name KTEmlHP3k

#######sh policy-map

Policy Map VOICE-WAN

Class VOICE

Strict Priority

Bandwidth 500 (kbps) Burst 12500 (Bytes)

Class class-default

Flow based Fair Queueing

Bandwidth 0 (kbps)

exponential weight 9

class min-threshold max-threshold mark-probablity

----------------------------------------------------------

0 - - 1/10

1 - - 1/10

2 - - 1/10

3 - - 1/10

4 - - 1/10

5 - - 1/10

6 - - 1/10

7 - - 1/10

rsvp - - 1/10

does the max-reserved-bandwidth give more bandwidth to the default class?

mounir.mohamed Wed, 07/16/2008 - 09:56

By default free or unallocated bandwidth available for overhead and best-effort traffic which is handled by the default-class (class-default) so Yes if you use this command the reset of bandwidth will increased and so class-default will use more bandwidth.

rll3@visto.com Wed, 07/16/2008 - 10:16

okay, that's great...

Now, as noted above in the sh class-map command there is no bandwidth assigned to the default class. Does this mean that the default class is working or not?

Correct Answer
JosephDoherty Wed, 07/16/2008 - 15:55

Yes, default class should be working.

On many Cisco routers, bandwidth in the default class configures a FIFO queue. Without it, normal default is flow based fair-queue. Also on most routers, default class flow queues compete for bandwidth with defined classes. However, the LLQ, when there's congestion, preempts all the other queues (up to its defined cap).

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