Details of QoS IP ToS

IP packets include a Type of Service (ToS) byte. The ToS byte includes:

Precedence Bits: This element is a subset of the DSCP and is composed of the upper 3 bits of the ToS byte. When configured to do so, the switch uses the precedence bits to determine a priority for handling the associated packet. (The switch does not change the setting of the precedence bits.) Using the ToS Precedence bits to prioritize IPv4 packets relies on priorities set in upstream devices and applications.

The following figure, shows an example of the ToS byte in the header for an IPv4 packet, and illustrates the diffserv bits and precedence bits in the ToS byte. (Note that the Precedence bits are a subset of the Differentiated Services bits.)

How the switch uses the ToS configuration
Outbound port	ToS option:
Outbound port	802.1p (value = 0 - 7)	Differentiated services
IP packet sent out an untagged port in a VLAN	Depending on the value of the IP Precedence bits in the packet’s ToS field, the packet will go to one of eight outbound port queues in the switch. See the table in Overview of QoS settings.	For a given packet carrying a ToS codepoint that the switch has been configured to detect: Change the codepoint according to the configured policy and assign the 802.1p priority specified for the new codepoint in the DSCP Policy Table (Differentiated Services Codepoint (DSCP) mapping). Do not change the codepoint, but assign the 802.1p priority specified for the existing codepoint in the DSCP Policy Table (Differentiated Services Codepoint (DSCP) mapping). Depending on the 802.1p priority used, the packet will leave the switch through a queue as defined in the table in Overview of QoS settings.If `No-override` (the default) has been configured for a specified codepoint, then the packet is not prioritized by ToS and, by default, is sent to the “normal priority” queue.
IP packet sent out an untagged port in a VLAN	Same as above, plus the IP Precedence value (0 - 7) will be used to set a corresponding 802.1p priority in the VLAN tag carried by the packet to the next downstream device. See the table below.	Same as above, plus the Priority value (0 - 7) will be used to set a corresponding 802.1p priority in the VLAN tag carried by the packet to the next downstream device. Where `No-override` is the assigned priority, the VLAN tag carries a “0” (normal priority) 802.1p setting if not prioritized by other QoS classifiers.

ToS IP-precedence bit mappings to 802.1p priorities
ToS byte IP precedence bits	Corresponding 802.1p priority	Service priority level
000	1	Lowest
001	2	Low
002	0	Normal
003	3
004	4
005	5
006	6
007	7	Highest