The assured forwarding (AF) model is used to provide priority values to different data applications.

The Expedited Forwarding (EF) model is used to provide resources to latency (delay) sensitive real-time, interactive traffic.
The EF model uses one marking — DSCP 46. DSCP 46 is backward compatible with an IP Precedence value of 5 as seen in the following binary pattern:

101110 = DSCP 46

The EF marking of 46 does NOT follow the drop preference rules of the assured forwarding model. Please do NOT think that the 11 means high drop preference. The EF model is used for voice over IP media traffic (RTP) by default in most vendors phones. Cisco IP Phones mark signaling packets (SCCP or SIP) to CS3 (24), while media (RTP) is marked to EF (DSCP 46) by default. All EF traffic is normally mapped to the priority queue (PQ) on Cisco switches and routers. The priority queue guarantees three critical services:

• Packet Loss
• Delay
• Jitter (delay variation)

The three most significant bits of 101 are only considered if IP Precedence was being used.

The Assured Forwarding (AF) PHB group provides delivery of IP packets in four
independently forwarded AF classes.
Within each AF class, an IP packet can be assigned one of three different
levels of drop precedence.

There are four classes defined in the Assured Forwarding model.

They are as follows:

Class 1 = 001
Class 2 = 010
Class 3 = 011
Class 4 = 100

Each class has three drop preferences (low, medium, and high).
The drop preferences are indicated by the next two bits of the
DSCP value in the ToS byte.

00 = No drop preference (class selector or user defined)
01 = Low Drop Preference
10 = Medium Drop Preference
11 = High Drop Preference

In case of congestion, the drop precedence of a packet determines the relative
importance of the packet within the AF class.
A congested DS node tries to protect packets with a lower drop precedence
value from being lost by preferably discarding packets with a higher drop
precedence value.

Expedited Forwarding (EF) PHB: “The EF PHB can be used to build a low loss, low latency, low jitter, assured bandwidth, end-to-end service through DS (Diffserv) domains. Such a service appears to the endpoints like a point-to- point connection or a “virtual leased line.” This service has also been described as Premium service.” Codepoint 101110 is recommended for the EF PHB, which corresponds to a DSCP value of 46.

(config-cmap)# match ip dscp ?
<0-63>   Differentiated services codepoint value
af11     Match packets with AF11 dscp (001010)
af12     Match packets with AF12 dscp (001100)
af13     Match packets with AF13 dscp (001110)
af21     Match packets with AF21 dscp (010010)
af22     Match packets with AF22 dscp (010100)
af23     Match packets with AF23 dscp (010110)
af31     Match packets with AF31 dscp (011010)
af32     Match packets with AF32 dscp (011100)
af33     Match packets with AF33 dscp (011110)
af41     Match packets with AF41 dscp (100010)
af42     Match packets with AF42 dscp (100100)
af43     Match packets with AF43 dscp (100110)
cs1      Match packets with CS1(precedence 1) dscp (001000)
cs2      Match packets with CS2(precedence 2) dscp (010000)
cs3      Match packets with CS3(precedence 3) dscp (011000)
cs4      Match packets with CS4(precedence 4) dscp (100000)
cs5      Match packets with CS5(precedence 5) dscp (101000)
cs6      Match packets with CS6(precedence 6) dscp (110000)
cs7      Match packets with CS7(precedence 7) dscp (111000)
default  Match packets with default dscp (000000)
ef       Match packets with EF dscp (101110)

Each DSCP is represented by its AF value, its value in decimal,
and its value in binary. Assured Forwarding Codepoints

Class 1

Low-Drop Precedence
AF11 = 10 (001010)

Medium-Drop Precedence
AF12 = 12 (001100)

High-Drop Precedence
AF13 = 14 (001110)

Class 2

Low-Drop Precedence
AF21 = 18 (010010)

Medium-Drop Precedence
AF22 = 20 (010100)

High-Drop Precedence
AF23 = 22 (010110)

Class 3

Low-Drop Precedence
AF31 = 26 (011010)

Medium-Drop Precedence
AF32 = 28 (011100)

High-Drop Precedence
AF33 = 30 (011110)

Class 4

Low-Drop Precedence
AF41 = 34 (100010)

Medium-Drop Precedence
AF42 = 36 (100100)

High-Drop Precedence
AF43 = 38 (100110)

Any Diffserv-aware system can use the AF codepoint as a guide for providing
differentiated forwarding behaviors to different classes of traffic.

When these packets reach a Diffserv router, the router evaluates the packets’
codepoints along with DSCPs of other traffic in the queue. The router then
forwards or drops packets, depending on the available bandwidth and the
priorities that are assigned by the packets’ DSCPs.

Note that packets that are marked with the EF PHB are guaranteed bandwidth over
packets that are marked with the various AF PHBs.