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.

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René Jorissen

Co-owner and Solution Specialist at 4IP Solutions

René Jorissen works as Solution Specialist for 4IP in the Netherlands. Network Infrastructures are the primary focus. René works with equipment of multiple vendors, like Cisco, Aruba Networks, FortiNet, HP Networking, Juniper Networks, RSA SecurID, AeroHive, Microsoft and many more. René is Aruba Certified Edge Expert (ACEX #26), Aruba Certified Mobility Expert (ACMX #438), Aruba Certified ClearPass Expert (ACCX #725), Aruba Certified Design Expert (ACDX #760), CCNP R&S, FCNSP and Certified Ethical Hacker (CEF) certified. You can follow René on Twitter and LinkedIn.

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