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706 Chapter 10: LAN QoS

Example 10-6 The show qos info config Command (Continued)

Default CoS = 0

 

Queue and Threshold Mapping:

Queue Threshold

CoS

----- ---------

---------------

1

1

0 1

1

2

2 3

2

1

4 5

2

2

6 7

Rx

drop

thresholds:

Rx

drop

thresholds are disabled for untrusted ports.

Queue #

Thresholds - percentage (abs values)

-------

 

-------------------------------------

150% 60% 80% 100% Tx drop thresholds:

Queue # Thresholds - percentage (abs values)

------- -------------------------------------

140% 100%

2 40% 100% Tx WRED thresholds:

WRED feature is not supported for this port_type. Queue Sizes:

Queue # Sizes - percentage (abs values)

------- -------------------------------------

180%

220%

WRR Configuration of ports with speed 1000MBPS: Queue # Ratios (abs values)

------- -------------------------------------

1100

2255

QoS Flow on the Catalyst 6500

Now that you have an understanding of the various hardware options available on the Catalyst 6500 as they relate to QoS, we can begin to discuss the QoS flow as the frames are received and forwarded through the switch. Figure 10-6 shows the QoS flow overview.

Figure 10-6 QoS Flow Overview

 

 

 

 

 

 

 

 

 

 

 

 

 

Ethernet

 

 

 

 

 

Transmit

 

 

 

 

 

 

 

 

 

 

 

 

 

Egress

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Queue

 

 

 

 

 

 

 

 

 

 

 

 

 

Port

 

 

 

 

 

Frame Enters

 

 

Ethernet

 

 

PFC / PFC2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Ingress

 

 

or

 

 

 

 

 

 

 

 

 

CoS = 0 for All Traffic

Switch

Port

 

 

L2 Switching Engine

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Exiting the MSFC

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Multilayer Switch Feature

 

 

 

 

 

FlexWAN

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Module

 

 

 

 

 

 

 

 

 

 

 

 

 

Card (MSFC)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Interfaces

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Cisco Catalyst Switch QoS Features 707

The frame arrives at the switch Ethernet ingress port. The existing frame marking, Ethernet ingress queuing structure, and the current trust state of the ingress Ethernet port determine the level of service the frame receives. After the ingress queue and threshold have been determined, the ingress port forwards the frame to the Layer 2 switching engine, or the PFC Layer 3 switching engine if one is installed.

When the supervisor does not have a PFC, the Layer 2 switching engine forwards the packet to the egress Ethernet port. The egress switch port uses the CoS marking on the frame to determine the queue that is used for the frame as it exits the switch.

When the supervisor does have a PFC, the PFC consults its cache to determine whether it has a known path to the destination address. If the path exists in the cache, the PFC forwards the packet to the correct egress port. Because the PFC can operate at Layer 3, it has the capability to switch the packet between VLANs without losing the Layer 2 CoS markings, so the frame retains its CoS marking, and the packet retains its DSCP or IP precedence marking.

In the event that the PFC does not have the path in cache, the packet is forwarded to the MSFC to determine which Ethernet egress port to use. When the MFSC makes a routing decision, and determines the correct Ethernet egress port needed to reach the destination address, the PFC populates its cache with this information for future use. This means the first packet in a flow loses the CoS value, because the first packet is routed at Layer 3 across the MSFC. However, subsequent packets are switched by the PFC and retain their original marked values. After the egress Ethernet port has been identified, the packet is assigned to the proper queue and threshold indicated by the Layer 2 or Layer 3 markings on the packet.

Ingress Queue Scheduling

The first opportunity for applying any kind of QoS tool to an incoming frame occurs when the frame enters the switch. Even before the supervisor or PFC forwards the frame to the egress line card, ingress QoS features occur. For instance, ingress queue scheduling defines the treatment that a frame or packet receives as it enters the switch. If QoS has been enabled, and the ingress port is trusted, the switch uses receive queue drop thresholds to schedule the arriving frames. Figure 10-7 shows the treatment received by the frame or packet in the ingress queue.

The first variable encountered as the frame enters the switch is trust. This asks the questions “Where is our trust boundary? Do we want to trust the markings on a frame as it enters the switch on this port?” If the port is untrusted, there is no need to continue to classify the frame. The ingress port just applies the configured port QoS value, typically 0, and forwards the frame to the switching engine.

If the port is trusted, the next questions asked are “Is this an 802.1Q or ISL trunk port? Does the incoming frame have an 802.1Q or ISL header?” If this ingress Ethernet port is not a trunk port, the configured port CoS value is assigned to the frame. If this ingress Ethernet port is configured as a trunk port, the CoS value marked on the frame is trusted.

708 Chapter 10: LAN QoS

Figure 10-7 Ethernet Ingress Port Treatment

Frame

Enters

Switch Ethernet Ingress Port Classification, Marking,

Scheduling, and Congestion Avoidance

Port Set to

Yes

 

Apply

 

 

 

 

 

 

 

 

 

Port

 

 

 

 

 

Untrusted?

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CoS

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

No

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

ISL or

 

No

 

Apply

 

 

 

 

 

 

 

Port

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

802.1q

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CoS

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Yes

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Port Set to Yes

 

 

 

 

 

 

 

trust-ipprec?

 

 

 

 

 

 

 

 

 

 

 

 

No

 

 

 

Yes

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Port Set to

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

trust-dscp?

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1q4t port

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

(tail-drop thresholds)

 

 

 

 

No

 

 

 

 

 

 

 

 

 

 

 

100% for CoS 6 and 7

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

80% for CoS 4 and 5

 

 

 

 

Port Is Set to

 

 

 

 

 

60% for CoS 2 and 3

 

 

 

 

 

 

 

 

 

50% for CoS 0 and 1

 

 

 

 

 

 

 

 

 

 

 

 

 

trust-cos

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1q4t port

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

(tail-drop thresholds)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Strict Priority Queue

 

 

 

 

(Default Values

 

 

100% for CoS 5

 

 

 

 

 

 

Standard Queue

 

 

 

 

 

 

 

 

 

Shown)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

100% for CoS 6 and 7

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

80% for CoS 4 and 5

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

60% for CoS 2 and 3

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

50% for CoS 0 and 1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1q1q01 port

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

(tail-drop thresholds)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Strict Priority Queue

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

100% for CoS 5

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Standard Queue

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

100% for CoS 0, 1, 2, 3, 4, 6, 7

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

To

Switching Engine

Cisco Catalyst Switch QoS Features 709

The next question depends on the hardware installed in the switch. If a PFC has been installed, the switch has the capability to classify and mark traffic based on Layer 3 markings, and so the question is asked “Do we want to trust IP precedence or DSCP from packets entering this Ethernet ingress port?” If the answer is yes, the packet is forward to the PFC. If we do not want to trust the Layer 3 markings, the switch relies on the CoS value to determine the correct queue and threshold.

After the proper queue and threshold has been identified, the question is asked “Is there room in the ingress queue for this frame, or has the drop threshold for this CoS value been met?” If the threshold has been met, the frame is dropped, otherwise the frame is forwarded on to the switching engine. If a PFC is not installed in the Catalyst switch, the frame flows to the Layer 2 switching engine, otherwise the frame is passed to the Layer 3 switching engine (PFC).

Layer 2 Switching Engine QoS Frame Flow

The Layer 2 switching engine has few options for classification and marking because it operates on the Layer 2 network. Figure 10-8 shows the options available.

Figure 10-8 Layer 2 Switching Engine Classification and Marking

From

Ingress Port

Match

Destination No

MAC

Address /

VLAN?

Yes

Mark CoS

To Egress

Port

The Layer 2 switching engine receives the frame from the ingress Ethernet port. The only variable that the Layer 2 switching engine has is the destination MAC address / VLAN match. If the arriving/received frame matches this criteria, the CoS value can be overwritten. If the received frame does not match this criteria, the frame is forwarded to the egress Ethernet port, retaining the received CoS value.