QoS - effects of bandwidth interface command

The bandwidth parameter in the interface configuration mode of a Cisco router is used by upper-layer protocols (routing protocols such as OSPF and EIGRP, as well as QoS mechanisms, and others) to optimize their operation, so the assigned value is important. This includes the mechanisms that govern the way bursting takes place on circuits that have throughput limitation QoS mechanisms applied to them.

In a scenario where QoS mechanisms such as policing or shaping is applied, where there is a committed rate and a committed burst rate, to which of those values should the interface bandwidth be set at?

There are advantages and disadvantages to each choice, and these will often determine what you will do. Here are some general advantages and disadvantages for each choice:

1. Setting the bandwidth statement according to the committed rate:

   • Advantages:
     • Accurate representation of the guaranteed bandwidth, which helps in QoS configurations and calculations.
     • Ensures proper prioritization of traffic based on the committed rate, which can improve overall network performance.
     • Helps in avoiding potential congestion and packet drops during periods of high traffic, as the router is aware of the available committed bandwidth.
   • Disadvantages:
     • Underestimating the available bandwidth, as it does not account for the burstable bandwidth that may be available during periods of low traffic.
     • May not fully utilize the available bandwidth, especially during times when additional burstable bandwidth is available.

2. Setting the bandwidth statement to the actual port speed:

   • Advantages:
     • Allows for the full utilization of the available bandwidth, including any burstable bandwidth that may be available during periods of low traffic.
     • Can potentially improve network performance, as the router is aware of the maximum available bandwidth.
   • Disadvantages:
     • May lead to inaccurate QoS calculations and prioritization, as the router is not aware of the guaranteed committed bandwidth.
     • During periods of high traffic, the router may not be able to effectively manage congestion or packet drops, as it assumes the availability of the full port speed.

In general, it’s recommended to set the bandwidth statement according to the committed rate on a burstable circuit, as it provides a more accurate representation of the guaranteed bandwidth and helps to better manage traffic during periods of congestion. However, if you want to maximize the use of available bandwidth and are willing to accept potential inaccuracies in QoS calculations, you may opt to set the bandwidth statement to the actual port speed.

Links:

https://forum.networklessons.com/t/bandwidth-statements-on-burst-able-circuits/36611/2?u=lagapides

https://networklessons.com/quality-of-service

https://community.cisco.com/t5/routing/qos-question-interface-bandwidth-sbucommand/td-p/873176