Cisco Catalyst Switch QoS Features 717

Table 10-11 Supported Receive and Transmit Queues of the Ethernet Modules for the Catalyst 6500 (Continued)

Cisco Catalyst 4500/4000 QoS Features

The Catalyst 4500 and 4000 series switches are modular and scalable, making these switches an excellent choice for wiring-closet installations, branch offices, Layer 3 distribution, or Layer 3 core installations for small and medium-sized networks. The modular design enables you to configure the switch based on the needs of your network. The addition of inline power positions the Catalyst 4500 and 4000 series switch as a natural choice for supporting devices that can take advantage of this feature, such as IP telephony and wireless access points.

QoS configuration on a Catalyst 4500 or Catalyst 4000 series switch depends on the supervisor engine module installed in the switch. Four supervisor engine modules are currently available for the Catalyst 4500 and Catalyst 4000 series of switches. Table 10-12 lists the 4500/4000 Catalyst switches that support each supervisor engine model.

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Table 10-12 Catalyst 4500/4000 Supervisor Engine Matrix

Supervisor Engine I and II

The Supervisor Engine I and Supervisor Engine II modules use the Cisco Catalyst operating system.

With the Supervisor Engine I and Supervisor Engine II module, the Catalyst 4500 and 4000 series switches have a single receive queue serviced in a FIFO order. The transmit queue consists of two standard queues with a single drop threshold (2q1t). The two standard queues are scheduled on a round-robin basis. Admission to the queues is based on the 802.1p CoS value of the frame; however, the Catalyst switch with the Supervisor Engine I or the Supervisor Engine II module only examines 2 of the 3 CoS bits to determine the CoS value of a frame. This limitation means that CoS values must be mapped in pairs. To map a CoS value of 5 to the second queue, for instance, both CoS values of 4 and 5 must be mapped to Queue 2.

When QoS is enabled globally, but without any other configuration, CoS values 0 though 7 are assigned to Queue 1, whereas broadcast and multicast traffic is assigned to Queue 2. This can lead to performance degradation, because most user traffic (unicast traffic) ends up in a single queue. Therefore, it is useful to configure the switch to put some unicast traffic in the second queue. (The configuration required to remap CoS values to the second queue is discussed in the “CoS-to-Egress Queue Mapping for the Catalyst OS Switch” section of this chapter.) Table 10-13 lists the default queue assignment for the Catalyst 4500/4000 with a Supervisor I or II after QoS has been enabled.

Table 10-13 Catalyst 4500/4000 Supervisor I or II Default Queue Admission

Trust on a Catalyst 4500/4000 Supervisor I or II is based on the QoS state. If QoS is enabled, the switch trusts all CoS values received on all ports.

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Table 10-15 Catalyst 4500/4000 Supervisor III or IV Default Queue Admission (Continued)

By default, all ports on a catalyst 4500/4000 are in the untrusted state after QoS has been enabled. Trust is enabled on a per-port basis, allowing more granular QoS control over the Supervisor I or II Engines. The Supervisor III and IV also add the capability to extend trust to the ASICs in the IP Phones, allowing the switch to trust the IP Phones, without having to trust the attached PC.

Because the Catalyst 4500/4000 Supervisor III and IV use the Cisco IOS operating system, they have the capability to classify traffic based on standard and extended IOS ACLs. This allows the switch to define traffic flows based on characteristics other than CoS, IP DSCP, or IP precedence, such as TCP port number, source address, and destination address.

The Supervisor Engine IV introduces an additional QoS feature called dynamic buffer limiting (DBL). DBL manages misbehaving traffic flows by tracking the buffering for each traffic flow and limiting the flow if excessive demands are placed on a buffer. A misbehaving flow is defined as a flow that uses all available bandwidth, consumes switch buffers, fills output queues, and does not respond to congestion feedback such as Random Early Detection (RED). A flow is defined as a source IP address, destination IP address, IP protocol, Layer 4 Transmission Control Protocol/User Datagram Protocol (TCP/UDP) ports, and VLAN. If the buffer usage of a flow exceeds a dynamically computed buffer limit, DBL constrains the flow’s buffering until the flow reaches a lower threshold. DBL is implemented at wire speed on all ports in the Catalyst family with Supervisor Engine IV.

The Supervisor Engine III and Supervisor Engine IV modules offer more granular control of QoS than their predecessors, which makes the newer modules a preferred choice for networks that carry real-time applications. Table 10-16 lists the available QoS features of a Catalyst 4500/ 4000 with a Supervisor III of IV engine.

Table 10-16 Catalyst 4500/4000 Supervisor III or IV QoS Features