LAN Class of Service CoS

Many LAN switches today can mark and react to a Layer 2 3-bit field called the Class of Service (CoS) located inside an Ethernet header. The CoS field only exists inside Ethernet frames when 802.1Q or Inter-Switch Link (ISL) trunking is used. You can use the field to set 8 different binary values, which can be used by the classification features of other QoS tools, just like IP precedence and DSCP.

Figure 3-3 shows the general location of the CoS field inside ISL and 802.1P headers.

Figure 3-3 LAN CoS Fields

ISL User Field (1 byte)

Frame Type

802.1Q/P Header

ISL Header ISL Header (26 Bytes

Frame Type

ISL Header ISL Header (26 Bytes

Original Frame








802.1Q Tag Field (2 bytes)

The term CoS really refers to two different fields—a field inside either the 802.1Q trunking header, or a field inside the ISL header. The IEEE 802.1Q standard uses the 3 most-significant bits of the 2-byte Tag Control field, called the user-priority bits. The Cisco proprietary ISL specification uses the 3 least-significant bits from the 1-byte User field, which is found inside the ISL header's user field byte. In general conversation, and in the QoS courses from Cisco, the term CoS applies to either of these two fields.

When can CoS be marked, and when can it be useful for classification to a QoS tool? First of all, trunking with 802.1Q or ISL must be enabled before the CoS field even exists! Second, as soon as the packet experiences Layer 3 forwarding, either with a router or some Layer 3 switch, the old LAN header gets discarded—which means you lose the CoS field. After a CoS field has been created and populated with the desired markings, routers and switches have several QoS tools that can react to these markings. Consider, for instance, a typical trunking environment, as shown in Figure 3-4, where all LAN switches are only performing Layer 2 switching.


Figure 3-4 CoS—Trunking Locations in a Typical Network, Layer 2 Switches Only

Figure 3-4 CoS—Trunking Locations in a Typical Network, Layer 2 Switches Only

To mark the CoS bits, trunking must be used—and in Figure 3-4, trunking could be used on every Ethernet segment. Switches typically use trunking on the Ethernet segments to other switches, routers, and to IP Phones. Typically, switches do not need to use trunking on segments connected to PCs or servers. Because some networking cards have the capability to support 802.1Q or ISL trunking, however, servers and PCs can set the CoS bits.

NOTE Trunking requires a Fast Ethernet interface, or Gigabit, or 10 Gigabit—it is not supported over 10-Mbps Ethernet. This book does not distinguish among the different types of Ethernet upon each mention.

Both routers and switches use QoS tools that can react to the marked CoS bits. Cisco routers can indeed mark CoS bits for frames exiting an Ethernet interface that supports trunking. For instance, R3 could mark CoS 5 on a frame it forwards out its FA 0/0 interface. Other Cisco router QoS tools can react to the marked CoS bits on incoming frames as well. For instance, R3 could mark packets entering its FA0/0 interface with a particular DSCP value based on the incoming CoS value. Later in this chapter, you will see a sample configuration for class-based marking that performs both of these functions.

Cisco switches vary widely regarding their capabilities to set CoS bits and react to previously marked CoS bits. Switches can support marking of CoS, and more often today support marking of IP precedence and DSCP as well. LAN switches that do support QoS features generally perform output queuing, and sometimes input queuing, choosing queues based on CoS values. Congestion avoidance using Weighted Random Early Detection (WRED) is another typical switch QoS feature. In addition, some switches support policing tools, also based on CoS. Although campus QoS is not covered in depth on the QoS exams today, it is an important topic, particularly with converged voice, video, and data networks. Chapter 10, "LAN QoS," covers LAN QoS in additional depth.

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