Monday, October 15, 2018

networking: Class of Service

In part 1, I stated that CoS doesn't refer to just layer 2 tagging of frames, as is commonly believed. Rather, CoS is the implementation of QoS principles enacted at various layers of the OSI model.

CoS facilitates the prioritization of traffic flows over a common path.
  • a means to recognize and control different types of traffic
  • ability for application traffic to be considered more or less important
  • mechanism to manage congestion of traffic
IEEE 802.1p/Q at the Ethernet layer and DSCP at the IP layer are some of the most commonly utilized standards-based CoS mechanisms.

Layer 2 method of CoS: 802.1p/Q Priority Code Point

  • 3-bit field in the 802.1q tag, with a value between 0-7, used to differentiate / give priority to certain Ethernet traffic.
  • When configuring lldp med, setting the "priority" or PCP value to 5, sets the PCP flag to 101, which will give those Ethernet frames the highest priority.
  • Because 802.1p/Q is a Layer 2 (Ethernet) standard, it only applies to the Ethernet header. At every Layer 3 boundary (router hop), the Layer 2 header, including PCP parameters, are stripped and replaced with a new header for the next link. Thus, 802.1Q doesn’t guarantee end-to-end QOS.

Layer 3 method of CoS: DSCP - Differentiated Services or DiffServ


  • 6-bit field in an IP header, with a value between 0-64, used to differentiate / give priority to certain IP traffic.
  • When configuring lldp med, configuring the DSCP value to "46", sets the DSCP flag in the IP header to "101110", and Datagrams with this tag will have the highest priority.
  • Network devices MUST be configured to use existing CoS values or they may be overwritten.


An example configuration string from a Brocade / Ruckus switch:


lldp med network-policy application voice tagged vlan 30 priority 5 dscp 46 ports ethe 1/1/1 to 1/1/48

Part 1, 2


Resources Utilized:

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