Traffic shapers and policers use packet traffic descriptors to ensure adherence to the service level agreement in QoS. However,
when traffic flows from one hop to another in a network, headers added or removed at interim hops affect the packet bytes
being accounted for by QoS at each hop. When your end-user network measures the packet bytes to ensure they receive the payload
as agreed, these additional header bytes cause a discrepancy.
QoS overhead accounting provides the flexibility to operators to decide which header bytes can be excluded by the traffic
shaper and policer and which can be included, depending on the end user’s requirements and device capabilities, to meet the
committed payload in units of bytes.
For example, if the QoS commitment includes the additional header bytes, the overhead accounting feature allows your router
to account for this overhead and reduces the traffic policing and shaping rates accordingly. This is also called a positive accounting overhead.
If however, the committed rate doesn’t include the additional bytes, overhead accounting allows your router to adjust the
core stream traffic such that the traffic policing and shaping rates are increased. This is also called a negative accounting overhead.
To summarize, QoS overhead accounting enables the router to account for packet overhead when shaping and policing traffic
to a specific rate. This accounting ensures that the router runs QoS features on the actual bandwidth that the subscriber
traffic consumes.
Any interface that supports QoS policies supports overhead accounting.
Note |
You can enable user overhead accounting using the optional configuration of accounting user-defined
<overhead size in bytes> while attaching the service policy on the egress interface.
|
Overhead Accounting controls the type of overhead and packet length for statistics, policing shaping and queuing. The account
option can be specified with a service-policy when applying a policy to an interface. For bundle interfaces, the configured
accounting option is applied to all member interfaces.
The configured accounting option is available on ingress and egress policing, queuing, and statistics for CRS-MSC-140G. In
CRS-MSC-40G, the configured accounting option isn’t available for queuing.
This table shows the packet length used during QoS for various
accounting options on the MSC-140:
Configured Accounting Option
|
Policing
|
Queuing
|
Statistics
|
Default
|
layer-all
|
layer 2
|
layer 2
|
Layer2
|
layer 2
|
layer 2
|
layer 2
|
No Layer2
|
layer 3
|
layer 3
|
layer 3
|
This table shows the packet length used during QoS for various
accounting options on the MSC-40:
Configured Accounting Option
|
Ingress Policing
|
Ingress Queuing
|
Ingress Statistics
|
Egress Policing
|
Egress Queuing
|
Egress Statistics
|
Default
|
layer 2
|
layer 3
|
layer 2
|
layer 2
|
layer 2
|
layer 2
|
Layer2
|
layer 2
|
layer 3
|
layer 2
|
layer 2
|
layer 2
|
layer 2
|
No Layer2
|
layer 3
|
layer 3
|
layer 3
|
layer 3
|
layer 2
|
layer 3
|
Associated Commands
service-policy (overhead accounting)