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QOS -- Max Reserve Bandwidth

When configuring the Quality of Service (QoS) policies, adequate bandwidth must exist for all required applications. The sum of all bandwidth used on each major application represents the minimum bandwidth requirement for any given link. This should consume no more than 75 percent of the total bandwidth available on that link. When there is available bandwidth that the total amount intended (or configured) to be used, the router will not apply the service policy.

For manifestations of the problem, refer to the bolded lines in this example:

!
policy-map TSA-ATMhttp://www.kddi.com/english/business/service_domestic/intranet/ethernet/image/p_m_qos_01.gif
class TSA-LEVEL-1
bandwidth percent 50
queue-limit 100
class TSA-ATM-LEVEL-2
bandwidth percent 25
class TSA-ATM-LEVEL-3
bandwidth percent 15
class class-default
fair-queue
queue-limit 100
!
interface ATM1/IMA1.40 point-to-point
description TSA case collection connection (Local VPI/VCI 1/40, Remote VPI/VCI 1/50)
ip address 10.1.1.1 255.255.255.252
pvc TSA-ATM 1/40 
vbr-nrt 3000 1536 1
no ilmi manage
oam-pvc manage
encapsulation aal5snap
service-policy output TSA-ATM <----This is the line that will not be add by IOS
!
cdp enable
!

With the bandwidth percent configured, Cisco IOS® Software determines the percent based off the Sustainable Cell Rate (SCR) value in the vbr-nrt statement. In this case, the value is 1536k.

This is how bandwidth works:

  • 1536 times 50 percent = 768k
  • 1536 times 25 percent = 384k
  • 1536 times 15 percent = 230k
  • 768 plus 384 plus 230 = 1382

To add these percentages: 1536 times (50 plus 25 plus 15) percent = 1536 times 90 percent = 1382.

The router determines the available bandwidth to be used as being 75 percent of the SCR. In this case, the amount of bandwidth available would be 1536 times 768 = 1152. This is less than the bandwidth configured for use (1382). Therefore, the router will not apply the service policy.

For a workaround to this problem, issue the max-reserved-bandwidth command.

Refer to these examples:

Router#conf t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#int atm1/ima1.40
Router(config-subif)#pvc TSA-ATM 1/40
Router(config-if-atm-vc)#max-reserved-bandwidth 90
Router(config-if-atm-vc)#end
Router#sho que
Router#sho queue atm1/ima1.40
Interface ATM1/IMA1.40 VC 1/40 
Queueing strategy: weighted fair
Output queue: 0/512/100/0 (size/max total/threshold/drops)
Conversations 0/1/64 (active/max active/max total)
Reserved Conversations 0/0 (allocated/max allocated)
Available Bandwidth 1382 kilobits/sec
Router#
Router#sh run int atm1/ima1.40
Building configuration...
Current configuration : 266 bytes
!
interface ATM1/IMA1.40 point-to-point
description TSA Case Collection Connection (Local VPI/VCI 1/40, Remote VPI/VCI 1/50)
ip address 10.1.1.1 255.255.255.252
pvc TSA-ATM 1/40 
vbr-nrt 3000 1536 1
max-reserved-bandwidth 90
service-policy output TSA-ATM
!
end
Router#show policy-map int
Router#show policy-map interface atm1/ima1.40
ATM1/IMA1.40: VC 1/40 -
Service-policy output: TSA-ATM
Class-map: TSA-ATM-LEVEL-1 (match-all)
0 packets, 0 bytes
5 minute offered rate 0 bps, drop rate 0 bps
Match: access-group 110
Queueing
Output Queue: Conversation 73 
Bandwidth 50 (%)
Bandwidth 768 (kbps) Max Threshold 64 (packets)
(pkts matched/bytes matched) 0/0
(depth/total drops/no-buffer drops) 0/0/0
Class-map: TSA-ATM-LEVEL-2 (match-all)
0 packets, 0 bytes
5 minute offered rate 0 bps, drop rate 0 bps
Match: access-group 111
Queueing
Output Queue: Conversation 74 
Bandwidth 25 (%)
Bandwidth 384 (kbps) Max Threshold 64 (packets)
(pkts matched/bytes matched) 0/0
(depth/total drops/no-buffer drops) 0/0/0
Class-map: TSA-ATM-LEVEL-3 (match-all)
0 packets, 0 bytes
5 minute offered rate 0 bps, drop rate 0 bps
Match: access-group 112
Queueing
Output Queue: Conversation 75 
Bandwidth 15 (%)
Bandwidth 230 (kbps) Max Threshold 64 (packets)
(pkts matched/bytes matched) 0/0
(depth/total drops/no-buffer drops) 0/0/0
Class-map: class-default (match-any)
1 packets, 32 bytes
5 minute offered rate 0 bps, drop rate 0 bps
Match: any 
Queueing
Flow Based Fair Queueing
Maximum Number of Hashed Queues 64 
(total queued/total drops/no-buffer drops) 0/0/0
Router#

For more informaton on the max-reserved-bandwidth command, refer to Understanding the max-reserved-bandwidth Command on ATM PVCs.

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