6.2.a Implement and troubleshoot end-to-end QoS

6.2.a [i] CoS and DSCP mapping

Differentiated Services (DiffServ) is a model in which traffic is treated by intermediate systems with relative priorities based on the type of services (ToS) field. DiffServ increases the number of definable priority levels by reallocating bits of an IP packet for priority marking.

The DiffServ architecture defines the DiffServ (DS) field, which supersedes the ToS field in IPv4 to make per-hop behavior (PHB) decisions about packet classification and traffic conditioning functions, such as metering, marking, shaping, and policing.

The RFCs do not dictate the way to implement PHBs; this is the responsibility of the vendor. Cisco implements queuing techniques that can base their PHB on the IP precedence or DSCP value in the IP header of a packet. Based on DSCP or IP precedence, traffic can be put into a particular service class.

Packets within a service class are treated the same way. AF43 (100110) and AF11 (001010) are the highest and lowest priority classes respectively that can be configured using DSCP encoding.

The six most significant bits of the DiffServ field is called as the DSCP. The last two Currently Unused (CU ) bits in the DiffServ field were not defined within the DiffServ field architecture; these are now used as Explicit Congestion Notification (ECN) bits. Routers at the edge of the network classify packets and mark them with either the IP Precedence or DSCP value in a DiffServ network. Other network devices in the core that support DiffServ use the DSCP value in the IP header to select a PHB behavior for the packet and provide the appropriate QoS treatment. COS 3 and 4 are mapped to DSCP 24 and 32 respectively.

The DSCP can be set to a desired value at the edge of the network in order to make it easy for core devices to classify the packet as shown in the Packet Classification section and provide a suitable level of service. Class-Based Packet Marking can be used to set the DSCP value.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 5610-5619).  . Kindle Edition.

http://www.cisco.com/c/en/us/support/docs/quality-of-service-qos/qos-packet-marking/10103-dscpvalues.html