Specifications

CHAPTER 8

Voice and Video in a Campus Network

n Dropping: Normally interface queues accept packets until they are full and then drop everything after that. You can

implement prioritized dropping so that less important packets are dropped before more important ones, such as with

Weighted Random Early Detection (WRED).

DSCP Values

Differentiated services provide levels of service based on the value of certain bits in the IP header or the 802.1Q tag.

Each hop along the way must be configured to treat the marked traffic the way you want; this is called per-hop behavior

(PHB).

In the Layer 2 802.1q tag, you use the three 802.1p bits to set the CoS value. Voice is usually set to 5 and video to 4.

In the Layer 3 IP header, you use the 8-bit ToS field. You can set either IP Precedence using the top 3 bits or

Differentiated Services Code Points (DSCP) using the top 6 bits of the field. The bottom 2 bits are set aside for conges-

tion notification. The default DSCP value is 0, which corresponds to best-effort delivery.

The six DSCP bits can be broken down into two sections: The first 3 bits define the DiffServ Assured Forwarding (AF)

class, and the next 2 bits define the drop probability within that class. The sixth bit is 0 and unused. AF classes 1–4 are

defined, and within each class, 1 is low drop probability, 2 is medium, and 3 is high (meaning that traffic is more likely to

get dropped if there is congestion). These are shown in Table 8-1.

Table 8-1 DSCP Assured Forwarding Values

Low Drop Medium Drop High Drop

Class 1 AF11 AF12 AF13

Class 2 AF21 AF22 AF23

Class 3 AF31 AF32 AF33

Class 4 AF41 AF42 AF43

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CCNP SWITCH 642-813 Quick Reference by Denise Donohue

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