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New Member

CBWFQ question


I need to implement QoS on WAN links to ensure the implementation of a new centrally located email system does not have an impact on all existing WAN applications by taking all the bandwidth away from these apps. We have no QoS enabled at present. Therefore all existing apps are in the default class (BE 0). The new application can easily be classified by IP subnet.

What I would like to do is use CBWFQ and assign a minimum of 20% of WAN bandwidth to the email application and allow it to take additional bandwidth from the default class, BUT only if the default class has bandwidth available. As there are only 2 classes, this means the default class having a bandwidth of 80%.

Questions are:

1. Is this a valid configuration with CBWFQ?

2. If the email class queue becomes full, will this class try and take from the default queue, IF this queue is also full?

3. If the default queue is full can it then take spare (available) bandwidth from the email class queue?

I would really appreciate some help if anybody knows the answers.

Thanks in anticipation.

kind regards


New Member

Re: CBWFQ question


1. You also need to allocate some bw for ip routing (5%) and VoIP/Call-Signaling if you're using VoIP. If you're not running VoIP, you still need to allocate bw for ip routing though.

2. yes

3. yes

The remaining bw not used by the priority class will be distributed among other classes as needed.

Cisco's definition of bandwith vs priority command: " Although the bandwidth guarantees provided by the bandwidth and priority commands have been described with words like "reserved" and "bandwidth to be set aside", neither command implements a true reservation. In other words, if a traffic class is not using its configured bandwidth, any unused bandwith is shared among other classes".


New Member

Re: CBWFQ question


Thanks for your input.

Your answer to question 1, we are not running VoIP, but you mentioned I would have to allocate BW for routing, won't this be catered for within the default class queue (this will be the left over BW which will be 80%)?

Your answer to my question 2, are you saying that if the email queue is full and the Best Effort queue is also full (ie congestion), the email queue will still grab BW from the Best Effort queue and thereby degrade the service in the Best Effort queue.

Thanks again,



New Member

Re: CBWFQ question


You need to setup a separate class for IP routing. Leaving it using the default class will also work, but it's not the best practice to do so.

If both of your queues are full (email and be)and assuming that you've only 2 classes -that's 100% of your bw - then subsequent packets are more likely to be dropped. You can implement "random-detect dscp-based" so packets can be dropped randomly instead of tail-drop.

By default, cisco won't allow you to configure more than 75% of your bw, so if you want to use all of it, you'll need to use "max-reserved bandwith 100" under the interface to override it.

Take a look at this document, it'll help you to configure QoS the way Cisco recommended. Look for "Enterprise QoS Solution Reference Network Design Guide v3.3" toward the bottom of the list.



New Member

Re: CBWFQ question

this is valid configuration

All policy will apply during congestion and bandwidth applied for a protocol is grunted. Bandwidth during congestion.

if there is no congestion email traffic can go more than 20 % and other traffic can go more than 80 %.

check this link for more detail

New Member

Re: CBWFQ question

Thanks for reply.

To enable this to happen, must I therefore assign BE class with a bandwidth statement (eg 80%) or will this exist anyway because there is only one other queue (for email class) which is set for 20%(ie 100%-20% = 80% for Best effort class)?

Thanks amrk