CHAPTER 3
Packet Flow Through the CoS Process
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Packet Flow Through the CoS Process Overview on page 15
Packet Flow Through the CoS Process Overview
Perhaps the best way to understand Junos CoS is to examine how a packet is treated on
its way through the CoS process. This topic includes a description of each step and figures
illustrating the process.
The following steps describe the CoS process:
1. A logical interface has one or more classifiers of different types applied to it (at the
[edit class-of-service interfaces] hierarchy level). The types of classifiers are based
on which part of the incoming packet the classifier examines (for example, EXP bits,
IEEE 802.1p bits, or DSCP bits). You can use a translation table to rewrite the values
of these bits on ingress.
NOTE: You can only rewrite the values of these bits on ingress on the
Juniper Networks M40e, M120, M320 Multiservice Edge Routers, and T
Series Core Routers with IQE PICs. For more information about rewriting
the values of these bits on ingress, see Configuring ToS Translation Tables.
2. The classifier assigns the packet to a forwarding class and a loss priority (at the [edit
class-of-service classifiers] hierarchy level).
3. Each forwarding class is assigned to a queue (at the [edit class-of-service
forwarding-classes] hierarchy level).
4. Input (and output) policers meter traffic and might change the forwarding class and
loss priority if a traffic flow exceeds its service level.
5. The physical or logical interface has a scheduler map applied to it (at the [edit
class-of-service interfaces] hierarchy level).
At the [edit class-of-service interfaces] hierarchy level, the scheduler-map and
rewrite-rules statements affect the outgoing packets, and the classifiers statement
affects the incoming packets.
15Copyright © 2013, Juniper Networks, Inc.
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