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CB-WFQ Bandwidth Allocation

When configuring Quality of Service with CB-WFQ I am always puzzling to get the correct classes. When configuring CB-WFQ it is important to remember that the router does not allow the class queues to consume more than 75% of the total interface bandwidth. The remaining 25% are used for the default class as well as all non-IP packets, like routing protocols.

A quick example shows the problem. I have a router with a 10 Mbps Ethernet interface. I will add a service-policy to this interface and create a priority queue for voice traffic of 8 Mbps.

First I created the class-map and the policy-map:

class-map match-all VOIP
match ip dscp ef
!
policy-map policy-cbwfq
class VOIP
priority 8192

Next I try to apply the policy-map to the interface, but I receive an error-message like shown below:

Router(config)#int fa 0/0
Router(config-if)#service-policy output policy-cbwfq
I/f FastEthernet0/0 class VOIP requested bandwidth 8192 (kbps), available only 7500 (kbps)

The error message clearly tells me that I can use only 75% of the bandwidth for class queues. The router automatically adds a fair-queue configuration to the interface as a fallback.

fair-queue 64 256 256

You can increase the amount of bandwidth for reservation of class queues with the command max-reserved bandwidth. This gives you the opportunity to increase the reserved bandwidth to 90%.

Router(config-if)#no fair-queue
Router(config-if)#max-reserved-bandwidth 90
Router(config-if)#service-policy output policy-cbwfq

The service-policy is now accepted by the router.

Router#sh policy-map interface fa0/0 output
FastEthernet0/0

Service-policy output: policy-cbwfq

Class-map: VOIP (match-all)
0 packets, 0 bytes
5 minute offered rate 0 bps, drop rate 0 bps
Match: ip dscp ef (46)
Queueing
Strict Priority
Output Queue: Conversation 264
Bandwidth 8192 (kbps) Burst 204800 (Bytes)
(pkts matched/bytes matched) 0/0
(total drops/bytes drops) 0/0

Class-map: class-default (match-any)
23 packets, 2252 bytes
5 minute offered rate 0 bps, drop rate 0 bps
Match: any

A useful with more information (difference between bandwidth percent and bandwidth remaining percent) can be found here.