Medianet Reference Guide - Cisco

Chapter 4
Medianet QoS Design Considerations
Cisco QoS Toolset
policy-enforcement mechanism for that traffic type, where it receives predefined treatment (either
preferential or deferential). Such treatment can include marking/remarking, queuing, policing,
shaping, or any combination of these (and other) actions.
• Marking, on the other hand, refers to changing a field within the packet to preserve the classification
decision that was reached. Once a packet has been marked, a “trust-boundary” is established on
which other QoS tools later depend. Marking is only necessary at the trust boundaries of the network
and (as with all other QoS policy actions) cannot be performed without classification. By marking
traffic at the trust boundary edge, subsequent nodes do not have to perform the same in-depth
classification and analysis to determine how to treat the packet.
Cisco IOS software performs classification based on the logic defined within the class map structure
within the Modular QoS Command Line Interface (MQC) syntax. MQC class maps can perform
classification based on the following types of parameters:
• Layer 1 parameters—Physical interface, sub-interface, PVC, or port
• Layer 2 parameters—MAC address, 802.1Q/p Class of Service (CoS) bits, MPLS Experimental
(EXP) bits
• Layer 3 parameters—Differentiated Services Code Points (DSCP), IP Precedence (IPP), IP Explicit
Congestion Notification (IP ECN), source/destination IP address
• Layer 4 parameters—TCP or UDP ports
• Layer 7 parameters—Application signatures and URLs in packet headers or payload via Network
Based Application Recognition (NBAR)
NBAR is the most sophisticated classifier in the IOS tool suite. NBAR can recognize packets on a
complex combination of fields and attributes. NBAR deep-packet classification engine examines the
data payload of stateless protocols and identifies application-layer protocols by matching them against
a Protocol Description Language Module (PDLM), which is essentially an application signature. NBAR
is dependent on Cisco Express Forwarding (CEF) and performs deep-packet classification only on the
first packet of a flow. The rest of the packets belonging to the flow are then CEF-switched. However, it
is important to recognize that NBAR is merely a classifier, nothing more. NBAR can identify flows by
performing deep-packet inspection, but it is up to the MQC policy-map to define what action should be
taken on these NBAR-identified flows.
Marking tools change fields within the packet, either at Layer 2 or at Layer 3, such that in-depth
classification does not have to be performed at each network QoS decision point. The primary tool within
MQC for marking is Class-Based Marking (though policers-sometimes called markers-may also be used,
as is discussed shortly). Class-Based Marking can be used to set the CoS fields within an 802.1Q/p tag
(as shown in Figure 4-10), the Experimental bits within a MPLS label (as shown in Figure 4-11), the
Differentiated Services Code Points (DSCPs) within an IPv4 or IPv6 header (as shown in Figure 4-12),
the IP ECN Bits (also shown in Figure 4-12), as well as other packet fields. Class-Based Marking, like
NBAR, is CEF-dependant.
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