Cisco Systems 15327 Priority Mechanism in IP and Ethernet, IP Precedence and Differentiated Services Code Point

14-2
Ethernet Card Software Feature and Configuration Guide, R7.2
January 2009
Chapter 14 Configuring Quality of Service
Priority Mechanism in IP and Ethernet
the ML-Series card to provide different levels of treatment to the different services. The different levels
are defined through the service elements of bandwidth, including loss and delay. A service-level
agreement (SLA) is a guaranteed level of these service elements.
The QoS mechanism has three basic steps. It classifies types of traffic, specifies what action to take
against a type of traffic, and specifies where the action should take place. The following sections explain
how the ML-Series card accomplishes these steps for unicast traffic. QoS for priority-multicast traffic
and traffic with unknown destination addresses is handled with a different mechanism, detailed in the
“Understanding Multicast QoS and Priority Multicast Queuing” section on page 14-24.
Priority Mechanism in IP and Ethernet
For any QoS service to be applied to data, there must be a way to mark or identify an IP packet or an
Ethernet frame. When identified, a specific priority can be assigned to each individual IP packet or
Ethernet frame. The IP Precedence field or the IP Differentiated Services Code Point (DSCP) field
prioritizes the IP packets, and the Ethernet class of service (IEEE 802.1p defined class of service [CoS])
is used for the Ethernet frames. IP precedence and Ethernet CoS are further described in the following
sections.
IP Precedence and Differentiated Services Code Point
IP precedence uses the three precedence bits in the IPv4 header’s ToS (type of service) field to specify
class of service for each IP packet (IETF RFC 1122). The most significant three bits on the IPv4 ToS
field provides up to eight distinct classes, of which six are used for classifying services and th e remaining
two are reserved. On the edge of the network, the IP precedence is assigned by the client device or the
router, so that each subsequent network element can provide services based on the dete rmined policy or
the SLA.
IP DSCP uses the six bits in the IPv4 header to specify class of service for each IP packet (IETF RFC
2474). Figure 14-1 illustrates IP precedence and DSCP. The DSCP field classifies packets into any of
the 64 possible classes. On the network edge, the IP DSCP is assigned by the client device or the router,
so that each subsequent network element can provide services based on the determined policy or the
SLA.