How Junos Os Cos Works - Juniper JUNOS OS 10.3 - SOFTWARE Manual

Complete Software Guide for Junos
Junos OS CoS for EX Series Switches Overview

How Junos OS CoS Works

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OS for EX Series Ethernet Switches, Release 10.3
When a network experiences congestion and delay, some packets must be dropped.
Junos operating system (Junos OS) class of service (CoS) divides traffic into classes to
which you can apply different levels of throughput and packet loss when congestion
occurs. This allows packet loss to happen according to rules that you configure.
For interfaces that carry IPv4, IPv6, and MPLS traffic, you can configure Junos OS CoS
features to provide multiple classes of service for different applications. CoS also allows
you to rewrite the Differentiated Services code point (DSCP), IP precedence, 802.1p, or
EXP CoS bits of packets egressing out of an interface, thus allowing you to tailor packets
for the remote peers' network requirements. See "Understanding Using CoS with MPLS
Networks on EX Series Switches" on page 3167 for more information about CoS for MPLS
networks.
CoS provides multiple classes of service for different applications. You can configure
multiple forwarding classes for transmitting packets, define which packets are placed
into each output queue, and schedule the transmission service level for each queue.
In designing CoS applications, you must give careful consideration to your service needs
and thoroughly plan and design your CoS configuration to ensure consistency and
interoperability across all platforms in a CoS domain.
Because Juniper Networks EX Series Ethernet Switches implement CoS in hardware
rather than in software, you can experiment with and deploy CoS features without
affecting packet-forwarding and switching performance.
NOTE: CoS policies can be enabled or disabled on each interface of an EX
Series switch. Also, each physical and logical interface on the switch can
have custom CoS rules associated with it. When CoS is used in an MPLS
network, there are some additional restrictions. See "Understanding Using
CoS with MPLS Networks on EX Series Switches" on page 3167.
How Junos OS CoS Works on page 3146
Default CoS Behavior on EX Series Switches on page 3147
Junos OS CoS works by examining traffic entering at the edge of your network. The
switches classify traffic into defined service groups to provide the special treatment of
traffic across the network. For example, voice traffic can be sent across certain links, and
data traffic can use other links. In addition, the data traffic streams can be serviced
differently along the network path. As the traffic leaves the network at the far edge, you
can rewrite the traffic to meet the policies of the targeted peer.
To support CoS, you must configure each switch in the network. Generally, each switch
examines the packets that enter it to determine their CoS settings. These settings then
dictate which packets are transmitted first to the next downstream switch. Switches at
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