698 Chapter 10: LAN QoS

This allows the network administrator to mark the packet as close to the end of the network without trusting all the applications on the PCs. You can gain more granular control of QoS tools by Specifying which devices are trusted.

Cisco Catalyst Switch QoS Features

Cisco Catalyst switches offer a wide array of QoS tools to help configure QoS in the LAN to support real-time applications. The following sections discuss the QoS capabilities, and hardware components if applicable, of these Cisco Catalyst switches:

Catalyst 6500 QoS Features

This section is not intended to be an exhaustive look at the Cisco Catalyst 6500. Although many options exist for this product, the scope of this discussion covers only the Qos features of the Catalyst 6500 as they relate to the prioritization of real-time applications such as IP telepehony.

The 6500 architecture distributes the QoS features among the line cards, the Supervisor card, and the Policy Feature Card (PFC). To appreciate what is really happening in the 6500, you must first have a basic understanding of the line cards and Supervisor cards.

Supervisor and Switching Engine

The type of supervisor and switching engine installed in your Catalyst 6500 determines the QoS feature set. As of this writing, there are four supervisor/switching engine combinations. Table 10-4 outlines these.

Table 10-4 Supervisor and Switching Engine Combinations

Cisco Catalyst Switch QoS Features 701

Example 10-4 show module of Catalyst 6500 with a Supervisor IA and Layer 2 Switching Engine (Continued)

Policy Feature Card

Cisco 6500 QoS features differ based on whether a PFC is installed and on other configuration options. For instance, the Layer 2 switching engine on the Supervisor I or Supervisor IA classifies, marks, and schedules traffic based on destination Media Access Control (MAC) address or VLAN tag information. Without the optional PFC, all marking of packets by the Layer 2 switching engine takes place in the CoS field. A Layer 2 switching engine does not examine the IP header, and so markings in the IP Precidence and DSCP fields are ignored.

With the addition of the PFC, the Catalyst 6500 is capable of enabling advanced QoS tools such as packet classification and marking, scheduling, and congestion avoidance based on either Layer 2, 3, or 4 header information. Specifically, the PFC can perform classification and marking, and policing. The line cards perform queuing and packet-drop logic. The PFC performs the actual packet-forwarding process, effectively making it a Layer 3 switching engine. The PFC can examine the Layer 2 and Layer 3 headers as the packet arrives. If the destination IP adress exists in the flow cache, for PFC cards, or the Cisco Express Forwarding (CEF) Forwarding Information Base (FIB), for PFC2 cards, the PFC can rewrite the following five header fields:

The Layer 2 (MAC) checksum (also called the frame checksum or FCS) allows the packet to retain the original Layer 2 and Layer 3 QoS markings.

In the event that the IP destination does not exist in the flow cache, or CEF FIB, the first packet of the flow is forwarded to the Multilayer Switch Feature Card (MSFC) to route the packet to the correct interface. The routing decision populates the flow cache or CEF FIB for subsequent