Now Chapter 2 examines the basic building blocks of a Junos CoS configuration, and then shows the packet flow through the various QoS functions, which are (almost) universal in Junos routing, switching, and security platforms. It then maps those QoS functions onto the packet flow through some of the different Junos hardware platforms, focusing on a few current platforms.TIP Remember that each network device behaves more or less independent-ly of all other network devices, so the only things you can actively influence are per-hop behaviors.The Building Blocks of a Junos CoS ConfigurationIn subsequent sections, this book focuses on Junos CoS as implemented on the M/T Series Routing Nodes, the MX Series Ethernet Services Routers, and the SRX Security Nodes. These are simply used as current examples of core, edge, and security nodes.In every Junos CoS implementation there are certain functions that are required in order to be able to influence the behavior of outbound packets on a particular interface. NOTEEach vendor’s networking equipment implements the control of these functions in different ways, and may use slightly different terminology. The terminology used in this book, and defined in this chapter, is the terminology used in Junos configurations, but the explanations should be sufficiently vendor-agnostic as to be broadly applicable to different vendors’ equipment.Let’s first list our key Junos CoS functions that can influence the behavior of outbound packets, and then devote a short section to each:Forwarding ClassClassification(s)PolicingRandom Early Discard (RED)ShapingSchedulingRemarking
Chapter 2: Basic Junos QoS Concepts and Packet Flow Through Routing Nodes13Forwarding ClassA forwarding class is a label, used entirely within a network node, which is used to identify all traffic that requires a single behavior when leaving that node. Forwarding classes do not explicitly appear outside a node, although if the QoS configuration of all nodes in a network is consistent, it can easily be derived from information in packet headers.ClassificationClassification is the act of identifying the class to which a packet belongs. It is usually initially performed on ingress to each node, although a packet may be reclassified at various points on its path through a network node.In Junos there are three main approaches to classifying packets, which vary in their degree of flexibility and in the complexity of the required configuration: Interface Based Classification, Behavior Aggregate (BA) Classification, and Multifield (MF) Classification. These approaches are not all mutually exclusive, and, in some combinations, can be applied in series to get a less granular first-pass behavior, followed by a more granular reclassification of a subset of the traffic.