CoS Applications

You can configure CoS features to meet your application needs. Because the components are generic, you can use a single CoS configuration syntax across multiple platforms. CoS mechanisms are useful for two broad classes of applications. These applications can be referred to as in the box and across the network.

In-the-box applications use CoS mechanisms to provide special treatment for packets passing through a single node on the network. You can monitor the incoming traffic on each interface, using CoS to provide preferred service to some interfaces (that is, to some customers) while limiting the service provided to other interfaces. You can also filter outgoing traffic by the packet’s destination, thus providing preferred service to some destinations.

Across-the-network applications use CoS mechanisms to provide differentiated treatment to different classes of packets across a set of nodes in a network. In these types of applications, you typically control the ingress and egress routing platforms to a routing domain and all the routing platforms within the domain. You can use JUNOS CoS features to modify packets traveling through the domain to indicate the packet’s priority across the domain.

Specifically, you modify the CoS code points in packet headers, remapping these bits to values that correspond to levels of service. When all routing platforms in the domain are configured to associate the precedence bits with specific service levels, packets traveling across the domain receive the same level of service from the ingress point to the egress point. For CoS to work in this case, the mapping between the precedence bits and service levels must be identical across all routing platforms in the domain.

JUNOS CoS applications support the following range of mechanisms:

• Differentiated Services (DiffServ)—The CoS application supports DiffServ, which uses 6-bit IPv4 and IPv6 header type-of-service (ToS) byte settings. The configuration uses CoS values in the IP and IPv6 ToS fields to determine the forwarding class associated with each packet.
• Layer 2 to Layer 3 CoS mapping—The CoS application supports mapping of Layer 2 (IEEE 802.1p) packet headers to routing platform forwarding class and loss-priority values.

Layer 2 to Layer 3 CoS mapping involves setting the forwarding class and loss priority based on information in the Layer 2 header. Output involves mapping the forwarding class and loss priority to a Layer 2-specific marking. You can mark the Layer 2 and Layer 3 headers simultaneously.

• MPLS EXP—Supports configuration of mapping of MPLS experimental (EXP) bit settings to routing platform forwarding classes and vice versa.
• VPN outer-label marking—Supports setting of outer-label EXP bits, also known as CoS bits, based on MPLS EXP mapping.

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