# IP Precedence and DSCP

## IP Precedence Standard IPP markings are similar to PCP: * 6 and 7 are reserved for 'network control' traffic (ie. OSPF messages between routers). * 5 = voice * 4 = video * 3 = voice signaling * 0 = best effort With 6 and 7 reserved, 6 possible values remain. Although 6 values is sufficient for many networks, the QoS requirements of some networks demand more flexibility. ## DSCP RFC 2474 (1998) defines the DSCP field, and other 'DiffServ' RFCs elaborate on its use. With IPP updated to DSCP, new standard markings had to be decided upon. * By having generally agreed upon standard markings for different kinds of traffic, QoS design & implementation is simplified, QoS works better between ISPs and enterprises, among other benefits. You should be aware of the following standard markings: * Default Forwarding (DF) - best effort traffic. * Expedited Forwarding (EF) - low loss/latency/jitter traffic (usually voice). * Assured Forwarding (AF) - A set of 12 standard values. * Class Selector (CS) - A set of 8 standard values, provides backward compatibility with IPP.

### DF (Default Forwarding)

**DF** is used for best-effort traffic. The DSCP marking for DF is 0. ### EF (Expedited Forwarding)

**EF** is used for traffic that requires low loss/latency/jitter. The DSCP marking for EF is 46. ### AF (Assured Forwarding) **AF** (Assured Forwarding) defines four traffic classes. All packets in a class have the same priority. Within each class there are three levels of drop *precedence*. * High drop precedence = more likely to drop the packet during congestion

{% hint style="info" %}

\ Formula to convert from AF value to decimal DSCP value: 8**X** + 2**Y** {% endhint %}

### CS (Class Selector) **CS** defines eight DSCP values for backward compatibility with IPP. The three bits that were added for DSCP are set to 0, and the original IPP bits are used to make 8 values.

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