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710 Chapter 10: LAN QoS

Layer 3 Switching Engine QoS Packet Flow

The PFC has the capability to classify, mark, and police traffic based on both Layer 2 and Layer 3 elements. The PFC also introduces the concept of QoS ACLs, which can be used for granular control over QoS configurations. Figure 10-9 illustrates the flow of a frame or packet through a PFC.

As the packet arrives at the PFC, the ingress interface or the ingress VLAN is examined to see whether an ACL is applied. If an ACL is not applied to either the ingress port or the ingress VLAN, the packet uses the default ACL. The default IP ACL consists of a single ACE that has a configurable marking rule and configurable policing rules. If the default ACL has not been configured, the ACE in the default ACL applies a DSCP value of 0 to all traffic passed from ingress ports configured with the untrusted port keyword. If an ACL is applied to the port or VLAN, the ACL examines the ACEs to see whether any entries match the arriving packet.

The PFC uses a DSCP setting for each IP packet that it processes. The PFC asks itself, “Do we trust the DSCP, IP precedence, or CoS values on the frame?” If these values are not trusted, the DSCP value specified in the default ACL is used. If these values are trusted, the switch uses the DSCP values received. For IP precedence and CoS values, the PFC uses an internal DSCP value generated from the IP precedence-to-DSCP or the CoS-to-DSCP mappings, respectively.

If policing has been enabled, the PFC determines the current traffic rate and decides to drop the packet, re-mark the packet, or place the packet in the egress queue unchanged.

If the packet is destined for the egress queue, the CoS value is derived from the DSCP value before the packet is delivered.

Egress Queue Scheduling

Egress queue scheduling defines the treatment that a frame or packet receives as it exits the switch. Figure 10-10 shows the treatment received by the frame or packet in the egress queue.

As the frame arrives at the egress queue, the CoS value is examined to determine the proper queue and threshold for the egress interface type. In this example, the switch has three different queues that a frame can be placed in: 2q2t, 1p2q2t, and 1p3q1t. The CoS value determines the proper egress queue. After the proper queue and threshold has been identified, the question is asked “Is there room in the egress 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 placed in a queue to await transport.

Cisco Catalyst Switch QoS Features 711

Figure 10-9 Layer 3 Switching Engine Classification, Marking, and Policing

From Ingress

Port or VLAN

 

 

L3 Switching Engine (PFC) Classification, Marking, and Policing

No

ACL(s)

 

IP

Yes

1Trust

Yes

Use Received

on

 

Received

 

 

Packet?

 

 

DSCP

 

Interface?

 

 

DSCP?

 

 

 

 

 

 

 

 

Yes

 

No

 

No

 

 

 

 

 

 

 

 

 

Match

Yes

 

 

1Trust

Yes

2Set DSCP

 

 

 

Received IP

 

ACE in ACL?

 

 

 

Precedence?

 

from Received

 

 

 

 

 

 

IP Precedence

 

 

 

 

 

 

 

 

No

 

 

 

No

 

 

 

Use Default

 

 

 

1Trust

 

 

 

ACL

 

 

 

Yes

3Set DSCP

 

 

 

 

 

Received or

from Received

 

 

 

 

 

Port CoS?

 

 

 

 

 

 

 

or Port CoS

 

 

 

 

 

 

 

 

 

 

 

 

No

 

 

 

 

 

 

 

1Trust Is from

 

Use DSCP

 

 

 

 

 

an Untrusted

 

from

 

 

 

 

 

Port

 

ACE

 

 

No

Markdown?

Yes

Out of

Yes

Policing

 

 

 

 

Profile?

 

Rule in

 

 

 

 

 

 

ACE?

 

 

 

 

 

 

 

 

 

 

Yes

 

No

 

No

 

 

Drop

4Set DSCP

 

 

 

5Derive CoS

 

 

Packet

to Marked

 

 

 

from DSCP

 

 

 

Down Value

 

 

 

 

1Specified by ACE Keyword or by

Port Keyword and dscp ACE Keyword To Egress Interface

2From IP Precedence-to-DSCP Map

3From CoS-to-DSCP Map

4From DSCP Markdown Map

5From DSCP-to-CoS Map

712 Chapter 10: LAN QoS

Figure 10-10 Ethernet Egress Port Treatment

From Switching

Engine or MSFC

Ethernet Egress port Scheduling, Congestion Avoidance, and Marking

2q2t Port (tail-drop thresholds)

High Priority

Standard Queue 100% for CoS 6 and 7 80% for CoS 4 and 5

Low Priority

Standard Queue 100% for CoS 2 and 3 80% for CoS 0 and 1

1p2q2t Port

Strict Priority Queue

100% for CoS 5

High Priority

Standard Queue (WRED-drop thresholds)

70%:100% for CoS 6 and 7 40%:70% for CoS 4

Low Priority

Standard Queue (WRED-drop thresholds)

70%:100% for CoS 2 and 3 40%:70% for CoS 0 and 1

1p3q1t Port

Strict Priority Queue

100% for CoS 5

Standard Queues (WRED-drop or tail-drop thresholds)

High Priority 100% for CoS 6 and 7

Medium Priority 100% for CoS 2, 3 and 4

Low Priority 100% for CoS 0 and 1

IP Traffic Yes

Write ToS

from PFC?

 

 

 

Byte into

 

 

 

 

 

 

Packet

 

 

 

 

 

 

 

 

 

No

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

ISL or

Yes

Write CoS

 

 

 

into

802.1q?

 

 

 

 

 

 

Frame

 

 

 

 

 

 

 

 

 

No

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

(Default Values Shown)

Transmit

Frame