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Cisco Meraki Documentation

Using Packet Prioritization on a Traffic Shaping Rule

Overview

Traffic priority is a way of ensuring that specific applications or subnets are guaranteed a certain amount of the uplink bandwidth at all times. Traffic priorities only come into play when the network is using all of the pre-configured bandwidth on the uplink. Take note that this bandwidth can change when link aggregation is enabled because the uplink changes per stream based upon the ratio specified.  

Traffic Queues

Specifying a traffic shaping rule as High, Normal, Low guarantees a certain fraction of the uplink to each priority level. The ratios are as follows:

  • High       4/7
  • Normal   2/7
  • Low       1/7

 

By default, all traffic is marked as having a Normal priority level. Traffic shaping rules that are marked at the same priority level share the same fraction of their respective levels. For instance, if there are 5 traffic shaping rules marked as High priority on a 10Mbps pipe each rule would have access to ~1.1Mbps.

 

Each traffic rule supersedes each rule below it and the rules below it must strictly adhere to their fractional bandwidth limits. For instance, if there is a high traffic shaping rule but no low traffic shaping rules configured then the high priority traffic would have access to 5/7 of the available bandwidth on the uplink and normal traffic would have 2/7s. Additionally, if there are no high priority traffic shaping rules then normal priority traffic gets 6/7 of the bandwidth and low priority gets 1/7 of the uplink's bandwidth.

Low-Latency Queue

Finally, in addition to the above configurable priorities, there is an additional low-latency queue priority which is not user configurable.  This queue has a greater priority than all other queues and as such is serviced before all other queues.  Only packets with a DSCP value of 46 (EF - Expedited Forwarding, Voice) are placed into this queue.  This feature is available and enabled on all MX14+ networks.

Examples

Here are some examples on a 7 Mbps Uplink:

 

Case 1:

With 2 high priority rules each get 2Mbps for a total of 4Mbps

With 2 normal priority rules each get 1Mbps for a total of 2Mbps

With 2 low priority rules each get .5Mbps for a total of 1 Mbps

 

Case 2:

With 2 high priority rules each get 2Mbps for a total of 4Mbps

With 1 normal priority rule it gets 2Mbps for a total of 2Mbps

With 10 low priority rules each get .1Mbps for a total of 1 Mbps

 

Case 3:

With 2 high priority rules each get 3Mbps for a total of 6Mbps

With no normal priority rules there is a total of 0Mbps

With 5 low priority rules each get .2Mbps for a total of 1 Mbps

 

Case 4:

With 5 high priority rules each get 1Mbps for a total of 5Mbps

With 4 normal priority rules each get .5Mbps for a total of 2Mbps

With no low priority rules there is a total of 0Mbps

 

Case 5:

With no high priority rules there is a total of 0Mbps

With 3 normal priority rules each get 2Mbps for a total of 6Mbps

With 2 low priority rules each get .5Mbps for a total of 1Mbps

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