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
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