
- •QoS Overview
- •“Do I Know This Already?” Quiz
- •QoS: Tuning Bandwidth, Delay, Jitter, and Loss Questions
- •Foundation Topics
- •QoS: Tuning Bandwidth, Delay, Jitter, and Loss
- •Bandwidth
- •The clock rate Command Versus the bandwidth Command
- •QoS Tools That Affect Bandwidth
- •Delay
- •Serialization Delay
- •Propagation Delay
- •Queuing Delay
- •Forwarding Delay
- •Shaping Delay
- •Network Delay
- •Delay Summary
- •QoS Tools That Affect Delay
- •Jitter
- •QoS Tools That Affect Jitter
- •Loss
- •QoS Tools That Affect Loss
- •Summary: QoS Characteristics: Bandwidth, Delay, Jitter, and Loss
- •Voice Basics
- •Voice Bandwidth Considerations
- •Voice Delay Considerations
- •Voice Jitter Considerations
- •Voice Loss Considerations
- •Video Basics
- •Video Bandwidth Considerations
- •Video Delay Considerations
- •Video Jitter Considerations
- •Video Loss Considerations
- •Comparing Voice and Video: Summary
- •IP Data Basics
- •Data Bandwidth Considerations
- •Data Delay Considerations
- •Data Jitter Considerations
- •Data Loss Considerations
- •Comparing Voice, Video, and Data: Summary
- •Foundation Summary
- •QoS Tools and Architectures
- •“Do I Know This Already?” Quiz
- •QoS Tools Questions
- •Differentiated Services Questions
- •Integrated Services Questions
- •Foundation Topics
- •Introduction to IOS QoS Tools
- •Queuing
- •Queuing Tools
- •Shaping and Policing
- •Shaping and Policing Tools
- •Congestion Avoidance
- •Congestion-Avoidance Tools
- •Call Admission Control and RSVP
- •CAC Tools
- •Management Tools
- •Summary
- •The Good-Old Common Sense QoS Model
- •GOCS Flow-Based QoS
- •GOCS Class-Based QoS
- •The Differentiated Services QoS Model
- •DiffServ Per-Hop Behaviors
- •The Class Selector PHB and DSCP Values
- •The Assured Forwarding PHB and DSCP Values
- •The Expedited Forwarding PHB and DSCP Values
- •The Integrated Services QoS Model
- •Foundation Summary
- •“Do I Know This Already?” Quiz Questions
- •CAR, PBR, and CB Marking Questions
- •Foundation Topics
- •Marking
- •IP Header QoS Fields: Precedence and DSCP
- •LAN Class of Service (CoS)
- •Other Marking Fields
- •Summary of Marking Fields
- •Class-Based Marking (CB Marking)
- •Network-Based Application Recognition (NBAR)
- •CB Marking show Commands
- •CB Marking Summary
- •Committed Access Rate (CAR)
- •CAR Marking Summary
- •Policy-Based Routing (PBR)
- •PBR Marking Summary
- •VoIP Dial Peer
- •VoIP Dial-Peer Summary
- •Foundation Summary
- •Congestion Management
- •“Do I Know This Already?” Quiz
- •Queuing Concepts Questions
- •WFQ and IP RTP Priority Questions
- •CBWFQ and LLQ Questions
- •Comparing Queuing Options Questions
- •Foundation Topics
- •Queuing Concepts
- •Output Queues, TX Rings, and TX Queues
- •Queuing on Interfaces Versus Subinterfaces and Virtual Circuits (VCs)
- •Summary of Queuing Concepts
- •Queuing Tools
- •FIFO Queuing
- •Priority Queuing
- •Custom Queuing
- •Weighted Fair Queuing (WFQ)
- •WFQ Scheduler: The Net Effect
- •WFQ Scheduling: The Process
- •WFQ Drop Policy, Number of Queues, and Queue Lengths
- •WFQ Summary
- •Class-Based WFQ (CBWFQ)
- •CBWFQ Summary
- •Low Latency Queuing (LLQ)
- •LLQ with More Than One Priority Queue
- •IP RTP Priority
- •Summary of Queuing Tool Features
- •Foundation Summary
- •Conceptual Questions
- •Priority Queuing and Custom Queuing
- •CBWFQ, LLQ, IP RTP Priority
- •Comparing Queuing Tool Options
- •“Do I Know This Already?” Quiz
- •Shaping and Policing Concepts Questions
- •Policing with CAR and CB Policer Questions
- •Shaping with FRTS, GTS, DTS, and CB Shaping
- •Foundation Topics
- •When and Where to Use Shaping and Policing
- •How Shaping Works
- •Where to Shape: Interfaces, Subinterfaces, and VCs
- •How Policing Works
- •CAR Internals
- •CB Policing Internals
- •Policing, but Not Discarding
- •Foundation Summary
- •Shaping and Policing Concepts
- •“Do I Know This Already?” Quiz
- •Congestion-Avoidance Concepts and RED Questions
- •WRED Questions
- •FRED Questions
- •Foundation Topics
- •TCP and UDP Reactions to Packet Loss
- •Tail Drop, Global Synchronization, and TCP Starvation
- •Random Early Detection (RED)
- •Weighted RED (WRED)
- •How WRED Weights Packets
- •WRED and Queuing
- •WRED Summary
- •Flow-Based WRED (FRED)
- •Foundation Summary
- •Congestion-Avoidance Concepts and Random Early Detection (RED)
- •Weighted RED (WRED)
- •Flow-Based WRED (FRED)
- •“Do I Know This Already?” Quiz
- •Compression Questions
- •Link Fragmentation and Interleave Questions
- •Foundation Topics
- •Payload and Header Compression
- •Payload Compression
- •Header Compression
- •Link Fragmentation and Interleaving
- •Multilink PPP LFI
- •Maximum Serialization Delay and Optimum Fragment Sizes
- •Frame Relay LFI Using FRF.12
- •Choosing Fragment Sizes for Frame Relay
- •Fragmentation with More Than One VC on a Single Access Link
- •FRF.11-C and FRF.12 Comparison
- •Foundation Summary
- •Compression Tools
- •LFI Tools
- •“Do I Know This Already?” Quiz
- •Foundation Topics
- •Call Admission Control Overview
- •Call Rerouting Alternatives
- •Bandwidth Engineering
- •CAC Mechanisms
- •CAC Mechanism Evaluation Criteria
- •Local Voice CAC
- •Physical DS0 Limitation
- •Max-Connections
- •Voice over Frame Relay—Voice Bandwidth
- •Trunk Conditioning
- •Local Voice Busyout
- •Measurement-Based Voice CAC
- •Service Assurance Agents
- •SAA Probes Versus Pings
- •SAA Service
- •Calculated Planning Impairment Factor
- •Advanced Voice Busyout
- •PSTN Fallback
- •SAA Probes Used for PSTN Fallback
- •IP Destination Caching
- •SAA Probe Format
- •PSTN Fallback Scalability
- •PSTN Fallback Summary
- •Resource-Based CAC
- •Resource Availability Indication
- •Gateway Calculation of Resources
- •RAI in Service Provider Networks
- •RAI in Enterprise Networks
- •RAI Operation
- •RAI Platform Support
- •Cisco CallManager Resource-Based CAC
- •Location-Based CAC Operation
- •Locations and Regions
- •Calculation of Resources
- •Automatic Alternate Routing
- •Location-Based CAC Summary
- •Gatekeeper Zone Bandwidth
- •Gatekeeper Zone Bandwidth Operation
- •Single-Zone Topology
- •Multizone Topology
- •Zone-per-Gateway Design
- •Gatekeeper in CallManager Networks
- •Zone Bandwidth Calculation
- •Gatekeeper Zone Bandwidth Summary
- •Integrated Services / Resource Reservation Protocol
- •RSVP Levels of Service
- •RSVP Operation
- •RSVP/H.323 Synchronization
- •Bandwidth per Codec
- •Subnet Bandwidth Management
- •Monitoring and Troubleshooting RSVP
- •RSVP CAC Summary
- •Foundation Summary
- •Call Admission Control Concepts
- •Local-Based CAC
- •Measurement-Based CAC
- •Resources-Based CAC
- •“Do I Know This Already?” Quiz
- •QoS Management Tools Questions
- •QoS Design Questions
- •Foundation Topics
- •QoS Management Tools
- •QoS Device Manager
- •QoS Policy Manager
- •Service Assurance Agent
- •Internetwork Performance Monitor
- •Service Management Solution
- •QoS Management Tool Summary
- •QoS Design for the Cisco QoS Exams
- •Four-Step QoS Design Process
- •Step 1: Determine Customer Priorities/QoS Policy
- •Step 2: Characterize the Network
- •Step 3: Implement the Policy
- •Step 4: Monitor the Network
- •QoS Design Guidelines for Voice and Video
- •Voice and Video: Bandwidth, Delay, Jitter, and Loss Requirements
- •Voice and Video QoS Design Recommendations
- •Foundation Summary
- •QoS Management
- •QoS Design
- •“Do I Know This Already?” Quiz
- •Foundation Topics
- •The Need for QoS on the LAN
- •Layer 2 Queues
- •Drop Thresholds
- •Trust Boundries
- •Cisco Catalyst Switch QoS Features
- •Catalyst 6500 QoS Features
- •Supervisor and Switching Engine
- •Policy Feature Card
- •Ethernet Interfaces
- •QoS Flow on the Catalyst 6500
- •Ingress Queue Scheduling
- •Layer 2 Switching Engine QoS Frame Flow
- •Layer 3 Switching Engine QoS Packet Flow
- •Egress Queue Scheduling
- •Catalyst 6500 QoS Summary
- •Cisco Catalyst 4500/4000 QoS Features
- •Supervisor Engine I and II
- •Supervisor Engine III and IV
- •Cisco Catalyst 3550 QoS Features
- •Cisco Catalyst 3524 QoS Features
- •CoS-to-Egress Queue Mapping for the Catalyst OS Switch
- •Layer-2-to-Layer 3 Mapping
- •Connecting a Catalyst OS Switch to WAN Segments
- •Displaying QoS Settings for the Catalyst OS Switch
- •Enabling QoS for the Catalyst IOS Switch
- •Enabling Priority Queuing for the Catalyst IOS Switch
- •CoS-to-Egress Queue Mapping for the Catalyst IOS Switch
- •Layer 2-to-Layer 3 Mapping
- •Connecting a Catalyst IOS Switch to Distribution Switches or WAN Segments
- •Displaying QoS Settings for the Catalyst IOS Switch
- •Foundation Summary
- •LAN QoS Concepts
- •Catalyst 6500 Series of Switches
- •Catalyst 4500/4000 Series of Switches
- •Catalyst 3550/3524 Series of Switches
- •QoS: Tuning Bandwidth, Delay, Jitter, and Loss
- •QoS Tools
- •Differentiated Services
- •Integrated Services
- •CAR, PBR, and CB Marking
- •Queuing Concepts
- •WFQ and IP RTP Priority
- •CBWFQ and LLQ
- •Comparing Queuing Options
- •Conceptual Questions
- •Priority Queuing and Custom Queuing
- •CBWFQ, LLQ, IP RTP Priority
- •Comparing Queuing Tool Options
- •Shaping and Policing Concepts
- •Policing with CAR and CB Policer
- •Shaping with FRTS, GTS, DTS, and CB Shaping
- •Shaping and Policing Concepts
- •Congestion-Avoidance Concepts and RED
- •WRED
- •FRED
- •Congestion-Avoidance Concepts and Random Early Detection (RED)
- •Weighted RED (WRED)
- •Flow-Based WRED (FRED)
- •Compression
- •Link Fragmentation and Interleave
- •Compression Tools
- •LFI Tools
- •Call Admission Control Concepts
- •Local-Based CAC
- •Measurement-Based CAC
- •Resources-Based CAC
- •QoS Management Tools
- •QoS Design
- •QoS Management
- •QoS Design
- •LAN QoS Concepts
- •Catalyst 6500 Series of Switches
- •Catalyst 4500/4000 Series of Switches
- •Catalyst 3550/3524 Series of Switches
- •Foundation Topics
- •QPPB Route Marking: Step 1
- •QPPB Per-Packet Marking: Step 2
- •QPPB: The Hidden Details
- •QPPB Summary
- •Flow-Based dWFQ
- •ToS-Based dWFQ
- •Distributed QoS Group–Based WFQ
- •Summary: dWFQ Options

702 Chapter 10: LAN QoS
packets; however, the first routed packet loses the original CoS marking. All subsequent packets are switched by the PFC and retain their original Layer 2 and Layer 3 markings.
Although the PFC and PFC2 are both Layer 3 switching engines, they differ in a few ways, as listed in Table 10-5.
Table 10-5 PFC and PFC2 Differences
PFC |
PFC2 |
|
|
Available as an option on Supervisor Engine IA |
Premounted on all Supervisor Engine II modules |
only |
|
|
|
Performs Layer 2/3/4 services according to on a |
Required for Layer 2/3/4 services in a CEF-based |
flow-based architecture |
architecture |
|
|
Performs certain Cisco IOS features such as PBR, |
Performs certain Cisco IOS features such as PBR, |
standard and extended access lists, and reflexive |
unicast RPF, TCP intercept, standard and |
ACLs in hardware |
extended access lists, and reflexive ACLs in |
|
hardware, and incorporates significant |
|
performance improvements |
|
|
Centralized forwarding mechanism based on a |
Distributed forwarding mechanism based on a |
flow-caching mechanism |
distributed Cisco Express Forwarding (DCEF) |
|
architecture. Used for IP unicast and multicast |
|
traffic and Layer 2 forwarding |
|
|
After a flow has been established, the PFC has the capability to use a QoS access list to police traffic to reduce the flow of traffic to a predefined limit. Traffic in excess of that limit can be dropped or have the DSCP value in the frame marked down to a lower value.
A QoS access-control list (ACL), consisting of a list of acess-control entries (ACEs), defines a set of QoS rules that the PFC uses to process incoming frames. ACEs are similar to a router ACL. The ACE defines classification, marking, and policing criteria for an incoming frame. If an incoming frame matches the criteria set in the ACE, the QoS engine processes the frame.
Ethernet Interfaces
The QoS role of Ethernet interfaces in a Catalyst 6500 includes the scheduling of packets and congestion management, as well as providing inline power to support the addition of real-time applications such as IP telephony. It is important to understand the capabilities of the Ethernet modules in your Catalyst to properly provision your network for the addition of real-time applications. Table 10-6 illustrates the QoS advantage that the current generation
of Ethernet modules, typically the WS-X65xx series of cards, offers over the past generation of

Cisco Catalyst Switch QoS Features 703
Ethernet modules, typically the WS-X63xx series of cards. Table 10-6 shows a comparision of the 48-port Ethernet modules.
Table 10-6 Ethernet Buffers, Queues, and Thresholds
Ethernet |
Total Buffer |
RX Buffer |
TX Buffer |
|
|
Module |
Size |
Size |
Size |
RX Queue |
TX Queue |
|
|
|
|
|
|
WS-X6548-RJ-45 |
1116 KB |
28 KB |
1088 KB |
1p1q0t |
1p3q1t |
|
|
|
|
|
|
WS-X6348-RJ-45 |
128 KB |
16 KB |
112 KB |
1q4t |
2q2t |
|
|
|
|
|
|
The show port capabilities command enables you to display QoS information about a particular port or modules. Example 10-5 displays the QoS capabilities of a supervisor IA card and a WS-X6248-RJ-45 card.
Example 10-5 The show port capabilities Command
CAT6K> (enable) show port capabilities 1/1
Model |
|
WS-X6K-SUP1A-2GE |
|
|
|||
Port |
|
1/1 |
|
|
|
||
Type |
|
|
1000BaseSX |
||||
Speed |
1000 |
|
|
|
|||
Duplex |
|
|
full |
||||
Trunk encap type |
|
|
802.1Q,ISL |
||||
Trunk mode |
|
|
on,off,desirable,auto,nonegotiate |
||||
Channel |
|
|
yes |
||||
Broadcast suppression |
|
|
percentage(0-100) |
||||
Flow control |
|
|
receive-(off,on,desired),send-(off,on,desired) |
||||
Security |
|
|
yes |
||||
Dot1x |
|
|
yes |
||||
Membership |
|
|
static,dynamic |
||||
Fast start |
|
|
yes |
||||
|
|
|
|
|
|
|
|
QOS scheduling |
|
|
rx-(1p1q4t),tx-(1p2q2t) |
||||
|
CoS rewrite |
|
|
yes |
|
||
ToS rewrite |
|
|
DSCP |
||||
UDLD |
|
|
yes |
||||
Inline power |
|
|
no |
||||
AuxiliaryVlan |
|
|
no |
||||
SPAN |
|
|
source,destination |
||||
COPS port group |
1/1-2 |
|
|
|
|||
Link debounce timer |
|
|
yes |
||||
CAT6K> (enable) |
|
|
|
|
|
|
|
CAT6K> (enable) show port capabilities 2/1 |
|||||||
Model |
|
|
|
||||
WS-X6248-RJ-45 |
|||||||
Port |
|
2/1 |
|
|
|
||
Type |
|
|
10/100BaseTX |
||||
Speed |
|
|
auto,10,100 |
||||
Duplex |
|
|
half,full |
||||
Trunk encap type |
|
|
802.1Q,ISL |
||||
Trunk mode |
|
|
on,off,desirable,auto,nonegotiate |
continues

704 Chapter 10: LAN QoS
Example 10-5 The show port capabilities Command (Continued)
|
Channel |
yes |
|
|
Broadcast suppression |
percentage(0-100) |
|
|
Flow control |
receive-(off,on),send-(off) |
|
|
Security |
yes |
|
|
Dot1x |
yes |
|
|
Membership |
static,dynamic |
|
|
Fast start |
yes |
|
|
|
|
|
|
QOS scheduling |
rx-(1q4t),tx-(2q2t) |
|
|
CoS rewrite |
yes |
|
|
ToS rewrite |
DSCP |
|
|
UDLD |
yes |
|
|
Inline power |
no |
|
|
AuxiliaryVlan |
1..1000,1025..4094,untagged,dot1p,none |
|
|
SPAN |
source,destination |
|
|
COPS port group |
2/1-48 |
|
|
Link debounce timer |
yes |
|
|
CAT6K> (enable) |
|
|
|
|
|
|
The WS-X65xx series of Ethernet cards introduce a priority queue and extend the buffer sizes to better accommodate traffic during times of congestion. In a side-by-side comparison, it is plain to see that the WS-65xx series of Ethernet line cards are the better choice for QoS.
The uplink Gigabit Ethernet ports on the Supervisor I support a single receive queue with four thresholds (1q4t) and two transmit queues with two thresholds (2q2t). The uplink Gigabit Ethernet ports on the Supervisor IA and the Supervisor II cards include enhanced QoS features that provide an additional priority queue for both ingress (1p1q4t) and egress (1p2q2t) interfaces. These queues are serviced in a WRR method, except for the priority queue, which is always serviced as soon as frames have entered the queue. Table 10-7 lists the TX and RX queues supported by each supervisor engine.
Table 10-7 Supervior Queues and Thresholds
Supervisor |
RX Queue |
TX Queue |
|
|
|
Supervisor II |
1p1q4t |
1p2q2t |
(WS-X6K-SUP2-2GE) |
|
|
|
|
|
Supervisor Engine IA |
1p1q4t |
1p2q2t |
(WS-X6K-SUP1A-2GE) |
|
|
|
|
|
Supervisor Engine I |
1q4t |
2q2t |
(WS-X6K-SUP1-2GE) |
|
|
|
|
|
The addition of the priority queue on the Supervisor IA and Supervisor II offers an advantage for providing QoS for real-time applications over Supervisor I.

Cisco Catalyst Switch QoS Features 705
Ingress and egress scheduling are always based on the CoS value associated with the frame. By default, higher CoS values are mapped to higher queue numbers. CoS 5 traffic, typically associated with Voice over IP (VoIP) traffic, is mapped to the strict-priority queue, if present.
In addition to the different queues, each standard queue has one or more drop thresholds. There are two types of drop thresholds:
•Tail-drop thresholds
On ports with tail-drop thresholds, frames of a given CoS value are admitted to the queue until the drop threshold associated with that CoS value is exceeded; subsequent frames of that CoS value are discarded until the threshold is no longer exceeded.
If CoS 1 is assigned to Queue 1, threshold 2, for example, and the threshold 2 watermark is 60 percent, frames with CoS 1 are not dropped until Queue 1 is 60 percent full. All subsequent CoS 1 frames are dropped until the queue is less than 60 percent full.
•WRED-drop thresholds
On ports with Weighted Random Early Detection (WRED)-drop thresholds, frames of a given CoS value are admitted to the queue based on a random probability designed
to avoid buffer congestion. The probability of a frame with a given CoS being admitted to the queue or discarded depends on the weight and threshold assigned to that CoS value.
If CoS 2 is assigned to Queue 1, threshold 2, for example, and the threshold 2 watermarks are 40 percent (low) and 80 percent (high), frames with CoS 2 are not dropped until Queue 1 is at least 40 percent full. As the queue depth approaches 80 percent, frames with CoS 2 have an increasingly higher probability of being discarded rather than being admitted to the queue. When the queue is more than 80 percent full, all CoS 2 frames are dropped until the queue is less than 80 percent full. The frames that the switch discards when the queue level is between the low and high thresholds are picked at random, rather than on a per-flow or FIFO basis. This method works well with protocols such as TCP that are capable of adjusting to periodic packet drops by backing off and adjusting their transmission window size.
Enter the show qos info config [mod/port] command to determine whether your Catalyst switch port supports WRED. In Example 10-6, the show qos info config command shows that port 2/ 1 does not support WRED.
Example 10-6 The show qos info config Command
Cat6500> show qos info config 2/1
QoS setting in NVRAM:
QoS is enabled
Port 2/1 has 2 transmit queue with 2 drop thresholds (2q2t).
Port 2/1 has 1 receive queue with 4 drop thresholds (1q4t).
Interface type:vlan-based
ACL attached:
The qos trust type is set to untrusted.