QoS Details
This chapter explores, in detail, ways of choosing and configuring quality of
service. The way you classify and mark traffic, and the type of QoS policies
you implement, will depend on the policy location and types of network
traffic present.
Classification and Marking
Classification is the most basic Quality of Service (QoS) step—until traffic is
identified, it cannot be provided a unique level of service. Traffic is often
classified by application, source or destination IP address, or inbound interface.
After traffic is classified, an appropriate marking can be applied to it. The
location where traffic is marked defines a trust boundary. If the device that
marked the traffic is trusted, then that marking is passed through the network
and honored by each device. If that device is untrusted, then some trusted
network entity must re-mark the traffic.
Classification and marking should be done as close to the traffic source as
possible because they can be resource intensive. Marking at the end device,
such as an IP phone, is ideal. Otherwise, mark (or re-mark) traffic at the
access switch or distribution switch if necessary.
Layer 2 markings include 802.1Q Class of Service (CoS) and Multiprotocol
Label Switching (MPLS) experimental bits. Frame relay markings are different—
they include setting the Backward Explicit Congestion Notification (BECN)
bit, the Forward Explicit Congestion Notification (FECN) bit, or the Discard
Eligible (DE) bit in the frame relay header. Layer 3 markings include
Differentiated Services Code Point (DSCP) and IP precedence. After traffic
is classified and marked, other routers and switches in the network can be
configured to provide QoS to it.