notes: CBWFQ

 

- CBWFQ is used to reserve a guaranteed minimum bandwidth in the output queue based on each user defined class.
- CBWFQ supports 64 classes/queues.
- Drop policy is tail drop or WRED, and it is configurable per class.
- Scheduling within a single class:
     - FIFO on 63 classes.
     - FIFO or WFQ on the class-default class.
- The queueing strategy only comes into effect when there is congestion in the output queue.
- Class class-default needs “fair-queue” configured if “bandwidth” was not specified.
- Weights can be defined by specifying:
     -  Bandwidth {in kbps}: Absolute reservation based on the configured amount.
     -  Bandwidth Percent: Absolute reservation based on percentage of configured interface "bandwidth" of the link.
    -  Remaining Percent: Relative reservation based on what is available interface bandwidth, not the configured "bandwidth".
- The queueing strategy will be 'class-based queueing' as listed with "show interface" command.
- Classification is done through ACL's or by using NBAR.

NOTE: Don't forget to change the default max-reserved-bandwidth of 75% for the interface before applying the service-policy.
NOTE: "max-reserve-bandwidth" is only a configuration limitation.

CBWFQ supports multiple class maps (the number depends upon the platform) to classify traffic into its corresponding FIFO queues. Tail drop is the default dropping scheme of CBWFQ. You can use weighted random early detection (WRED) in combination with CBWFQ to prevent congestion of a class.

commands:

- Shows the policy map configured with all the counters

sh policy-map interface {int}

class-map SMTP
match access-group SMTP
class-map match-any HTTP
match protocol HTTP
class-map FTP
match access-group FTP

policy-map QoS
class SMTP
bandwidth 512
class HTTP
bandwidth percent 25
class FTP
bandwidth remaining percent 25
class class-default
fair-queue - Required if "bandwidth" was not specified

optional: using WRED

class class-default
random-detect [dscp-based] [precedence-based]

interface S0/0
bandwidth 1024
max-reserved-bandwidth {%} - Changes the default 75% reserved bandwidth used when queueing is applied.

interface s0/0

service-policy output QoS