Foundation Summary

The "Foundation Summary" is a collection of tables and figures that provide a convenient review of many key concepts in this chapter. For those of you already comfortable with the topics in this chapter, this summary could help you recall a few details. For those of you who just read this chapter, this review should help solidify some key facts. For any of you doing your final preparation before the exam, these tables and figures are a convenient way to review the day before the exam.

Table 4-18 outlines the key features of queuing tools, with a brief definition of each feature.

Table 4-18 Key Concepts When Comparing Queuing Tools

Table 4-18 outlines the key features of queuing tools, with a brief definition of each feature.

Table 4-18 Key Concepts When Comparing Queuing Tools

Feature

Definition

Characteristic Affected

Classification

The capability to examine packets to determine into which queue the packet should be placed. Many options are available.

None

Drop policy

When the queue has been determined, the drop policy defines the rules by which the router chooses to drop the packet. Tail drop, modified tail drop, WRED (Weighted Random Early Detect), and FRED (Flow-Based Random Early Detect) are the main options.

Loss

Scheduling inside a single queue

Inside a single queue, packets can be reordered. In most cases, however, FIFO logic is used for packets inside each queue.

Bandwidth, delay, jitter, and loss

Scheduling between different queues

The logic that defines how queuing chooses the next packet to take from the output queues and place it in the TX Queue (Transmit Queue).

Bandwidth, delay, jitter, and loss

Maximum number of queues

The maximum number of different queues the queuing tools support, which in turn implies the maximum number of traffic classifications that can be treated differently by the queuing method.

None

Maximum queue length

The maximum number of packets in a single queue.

Loss, delay

Figure 4-23 depicts the TX Queue, along with a single FIFO Queuing output queue.

Figure 4-23 Single FIFO Output Queue, with a Single TX Queue

4 Packets Arriving when TX ■ Queue Is Empty

TX Queue, Length 4, Not Controlled by Queuing Tool

Output Queue: Not Needed in This Case

TX Queue, Length 4, Not Controlled by Queuing Tool

Output Queue: Not Needed in This Case

Packet 4

Packet 3

Packet 2

Packet 1

Packets Arriving when TX Queue Is

Full, and Some-

Packets in Software Queue

TX Queue, Length 4, Not Controlled Output Queue: Controlled by Queuing Tool by Queuing Tool

Packet 7

Packet 6

Packet 5

Packet 4

Packet 3

Packet 2

Packet 1

Figure 4-24 shows how TX Queues affect queuing. With queuing configured with two queues, seven packets arrive, numbered in the order in which they arrive. The output queuing configuration specifies that the first two packets (1 and 2) should be placed into Queue 2, and the next four packets (numbered 3 through 6) should be placed into Queue 1.

Figure 4-24 Two Output Queues, with Scheduler Always Servicing Queue 1 Rather Than Queue 2 When Packets Are in Queue 1

Assumed Behavior if No TX Queue

6 Packets: First 2 to Queue 2,— Next 4 to Queue 1

Output Queue 1 — Preferred Queue

Output Queue 1 — Preferred Queue

Packet 6

Packet 5

Packet 4

Packet 3

Packet 2

Packet 1

Output Queue 2

Output Queue 2

Scheduler Would Take . Packets 3-6 from Queue 1 - Before Packets 1-2 from Queue 2

Actual Behavior with TX Queue

6 Packets: First 2 to Queue 2,— Next 4 to Queue 1

R1 — SerialO Output Queue 1

Packet 6

Packet 5

Packet 4

Packet 3

>-

TX Queue, Length 2

Packet 2 Packet 1

Packets Exit in Order They Arrived

The following list summarizes the key points about TX Rings and TX Queues in relation to their effect on queuing:

• The TX Queue/TX Ring always performs FIFO scheduling, and cannot be changed.

• The TX Queue/TX Ring uses a single queue, per interface.

• IOS shortens the interface TX Queue/TX Ring automatically when an output queuing method is configured

• You can configure the TX Ring/TX Queue length to a different value.

To delay the traffic, traffic shaping places the packets into the queue associated with the subinterface or DLCI and drains the traffic from the shaping queue at the shaped rate. Figure 4-25 shows the structure of the queues on a subinterface, interface, and the TX Queue, when shaping is enabled.

Figure 4-25 Shaping Queues, Interface Queues, and TX Ring

Routerl s0/0.1

Table 4-19 summarizes some of the key features of PQ.

Table 4-19 PQ Functions and Features

Table 4-19 summarizes some of the key features of PQ.

Table 4-19 PQ Functions and Features

PQ

Feature

Explanation

Classification

Classifies based on matching an ACL for all Layer 3 protocols, incoming interface, packet size, whether the packet is a fragment, and TCP and UDP port numbers.

Drop policy

Tail drop.

Maximum number of

4.

queues

Maximum queue length

Infinite; really means that packets will not be tail dropped, but will be queued.

Table 4-19 PQ Functions and Features (Continued)

PQ Feature

Explanation

Scheduling inside a single queue

FIFO.

Scheduling among all queues

Always service higher-priority queues first; result is great service for the High queue, with potential for 100% of link bandwidth. Service degrades quickly for lower-priority queues.

Table 4-20 summarizes some of the key features of CQ.

Table 4-20 CQ Functions and Features

Table 4-20 summarizes some of the key features of CQ.

Table 4-20 CQ Functions and Features

CQ Feature

Explanation

Classification

Classifies based on matching an ACL for all Layer 3 protocols, incoming interface, packet size, whether the packet is a fragment, and TCP and UDP port numbers.

Drop policy

Tail drop.

Number of queues

16.

Maximum queue length

Infinite; really means that packets will not be tail dropped, but will be queued.

Scheduling inside a single queue

FIFO.

Scheduling among all queues

Services packets from a queue until a byte count is reached; round-robins through the queues, servicing the different byte counts for each queue. The effect is to reserve a percentage of link bandwidth for each queue.

Flow-Based WFQ, or simply WFQ, classifies traffic into flows. Flows are identified by at least five items in an IP packet.

• Source IP address

• Destination IP address

• Transport layer protocol (TCP or UDP) as defined by the IP Protocol header field

• TCP or UDP destination port

For perspective on the sequence of events for WFQ, marking the sequence number, and serving the queues, examine Figure 4-26.

Figure 4-26 WFQ—Assigning Sequence Numbers and Servicing Queues

3) Maximum Number of Queues

4) Maximum Queue Length

5) Scheduling Inside Queue 6) Scheduler Logic

3) Maximum Number of Queues

4) Maximum Queue Length

5) Scheduling Inside Queue 6) Scheduler Logic

- Source/Destination Port

WFQ calculates the sequence number (SN) before adding a packet to its associated queue. The formula for calculating the SN for a packet is as follows:

Previous_SN + weight * new_packet_ength

Table 4-21 lists the weight values used by WFQ before and after the release of 12.0(5)T/12.1.

Table 4-21 Weight Values Used by WFQ

Table 4-21 lists the weight values used by WFQ before and after the release of 12.0(5)T/12.1.

Table 4-21 Weight Values Used by WFQ

Precedence

Before 12.0(5)T/12.1

After 12.0(5)T/12.1

0

4096

32384

1

2048

16192

2

1365

10794

3

1024

8096

4

819

64l6

5

682

539l

6

585

4626

l

512

4048

WFQ discards some packet when a queue's congestive discard threshold (CDT) has been reached. To appreciate how the CDT is used, examine Figure 4-27.

Figure 4-27 WFQ Modified Tail Drop and Congestive Discard Threshold

Figure 4-27 WFQ Modified Tail Drop and Congestive Discard Threshold

Tables 4-22 and 4-23 list the configuration and exec commands related to WFQ.

Table 4-22 Configuration Command Reference for WFQ

Command

Mode and Function

fair-queue [congestive-discard-threshold [dynamic-queues [reservable-queues]]]

Interface configuration mode; enables WFQ, sets the CDT, sets maximum number of queues, and sets the number reserved for RSVP use

hold-queue length {in | out}

Interface configuration mode; changes the length of the hold queue

Table 4-23 Exec Command Reference for WFQ

Command

Function

show queue interface-name interface-number [vc [vpi/] vci]]

Lists information about the packets that are waiting in a queue on the interface

show queueing [custom | fair | priority | random-detect [interface atm-subinterface [vc [[vpi/] vci]]]]

Lists configuration and statistical information about the queuing tool on an interface

Table 4-24 summarizes some of the key features of WFQ.

Table 4-24 summarizes some of the key features of WFQ.

Table 4-24 WFQ Functions and Features

WFQ Feature

Explanation

Classification

Classifies without configuration, based on source/destination IP address/ port, protocol type (TCP|UDP), and ToS.

Drop policy

Modified tail drop.

Number of queues

4096.

Maximum queue length

Congestive discard threshold per queue (max 4096), with an overall limit based on the hold queue for all queues (max 4096).

Scheduling inside a single queue

FIFO.

Scheduling among all queues

Serves lowest sequence number (SN). The SN is assigned when the packet is placed into the queue, as a function of length and precedence.

Table 4-25 summarizes some of the key features of CBWFQ.

Table 4-25 CBWFQ Functions and Features

Table 4-25 summarizes some of the key features of CBWFQ.

Table 4-25 CBWFQ Functions and Features

CBWFQ Feature

Description

Classification

Classifies based on anything that MQC commands can match, just like CB marking. Includes all extended IP ACL fields, NBAR, incoming interface, CoS, precedence, DSCP, source/destination MAC, MPLS Experimental, QoS group, and RTP port numbers

Drop policy

Tail drop or WRED, configurable per queue.

Number of queues

64.

Maximum queue length

64.

Scheduling inside a single queue

FIFO on 64 queues; FIFO or WFQ on class-default queue.

Scheduling among all queues

Algorithm is not published. The result of the scheduler provides a percentage guaranteed bandwidth to each queue.

All the commands for CBWFQ are repeated for reference in Tables 4-26 and 4-27.

Table 4-26 Command Reference for CBWFQ

Command

Mode and Function

class-map class-map-name

Global config; names a class map, where classification options are configured.

match ...

Class map subcommand; defines specific classification parameters.

Table 4-26 Command Reference for CBWFQ (Continued)

Command

Mode and Function

match access-group {access-group | name access-group-name}

Access-control list (ACL).

match source-address mac address

Source MAC address.

match ip precedence ip-precedence-value [ip-precedence-value ip-precedence-value ip-precedence-value]

IP precedence.

match mpls experimental number

MPLS Experimental.

match cos cos-value [cos-value cos-value cos-value]

CoS.

match destination-address mac address

Destination MAC address.

match input-interface interface-name

Input interface.

match ip dscp ip-dscp-value [ip-dscp-value ip-dscp-value ip-dscp-value ip-dscp-value ip-dscp-value ip-dscp-value ip-dscp-value]

IP DSCP.

match ip rtp starting-port-number port-range

RTP's UDP port number range.

match qos-group qos-group-value

QoS group.

match protocol protocol-name

NBAR protocol types.

match protocol citrix [app application-name-string].

NBAR Citrix applications.

match protocol http [url url-string | host hostname-string | mime MIME-type]

Host name and URL string.

match any

Matches any and all packets.

policy-map policy-map-name

Global config; names a policy, which is a set of actions to perform.

class name

Policy map subcommand; identifies the packets to perform QoS actions on by referring to the classification logic in a class map

bandwidth {bandwidth-kbps | percent percent}

Class subcommand; sets literal or percentage bandwidth for the class. Must use either use actual bandwidth or percent on all classes in a single policy map.

fair-queue [queue-limit queue-value]

Class subcommand; enables WFQ in the class (class-default only).

random-detect dscp dscpvalue min-threshold max-threshold [mark-probability-denominator]

Class subcommand; enables DSCP-based WRED in the class.

Table 4-26 Command Reference for CBWFQ (Continued)

Command

Mode and Function

random-detect precedence precedence min-threshold max-threshold mark-pmb-denominator

Class subcommand; enables precedence-based WRED in the class.

max-reserved-bandwidth percent

Interface subcommand; defines the percentage of link bandwidth that can be reserved for CBWFQ queues besides class-default.

Table 4-27 Exec Command Reference for CBWFQ

Command

Function

show policy-map policy-map-name

Lists configuration information about all MQC-based QoS tools

show policy-map interface-spec [input | output] [class class-name]

Lists statistical information about the behavior of all MQC-based QoS tools

To prevent LLQ from having the same problem as PQ, where packets in the highest-priority queue could dominate, LLQ's scheduler actually works as shown in Figure 4-28.

Figure 4-28 Servicing Queues with LLQ and CBWFQ—The Real Story

Figure 4-28 Servicing Queues with LLQ and CBWFQ—The Real Story

The single additional configuration command for LLQ is listed in Table 4-28.

Table 4-28 Command Reference for LLQ

Command

Mode and Function

priority{bandwidth-kbps | percent percentage} [burst]

Class subcommand; enables LLQ in this class, reserves bandwidth, and enables the policing function. The burst for the policer can also be configured with this command.

Table 4-29 summarizes the main features of IP RTP Priority, and compares the features with LLQ.

Table 4-29 Comparison of LLQ and IP RTP Priority Features

Feature

LLQ

IP RTP Priority

Adds a priority queue to WFQ

No

Yes

Adds a priority queue to CBWFQ

Yes

Yes

Can classify on even UDP ports in a specified range

Yes

Yes

Can classify on anything MQC can use to classify

Yes

No

Reserves a configured amount of bandwidth

Yes

Yes

Bandwidth is policed, so priority queue cannot exceed the configured bandwidth

Yes

Yes

Currently recommended best queuing tool for low latency

Yes

No

Table 4-30 lists the two configuration commands used with IP RTP Priority.

Table 4-30 Command Reference for IP RTP Priority

Table 4-30 lists the two configuration commands used with IP RTP Priority.

Table 4-30 Command Reference for IP RTP Priority

Command

Mode and Function

ip rtp priority starting-rtp-port-number port-number-range bandwidth

Interface configuration mode; enables IP RTP Priority on a subinterface or interface

frame-relay ip rtp priority starting-rtp-port-number port-number-range bandwidth

FRTS (Frame Relay traffic shaping) map-class configuration mode; enables IP RTP Priority in an FRTS map class, which is then enabled on a subinterface or DLCI (data-link connection identifier)

Table 4-31 summarizes the details of the scheduler, drop, and maximum queues supported for the queuing tools covered in detail in this chapter.

Table 4-31 Summary of Scheduler, Drop, and Number of Queues

Table 4-31 summarizes the details of the scheduler, drop, and maximum queues supported for the queuing tools covered in detail in this chapter.

Table 4-31 Summary of Scheduler, Drop, and Number of Queues

Tool

Scheduler

Drop Policy

Max # of Queues

FIFO

Services packets in the same order that they arrived.

Tail drop

1

PQ

Always services higher-priority queues first; the result is great service for the High queue, with potential for 100% of link bandwidth. Service degrades quickly for lower-priority queues.

Tail drop

4

Table 4-31 Summary of Scheduler, Drop, and Number of Queues (Continued)

Tool

Scheduler

Drop Policy

Max # of Queues

CQ

Services packets from a queue until a byte count is reached; round-robins through the queues, servicing the different byte counts for each queue. The effect is to reserve a percentage of link bandwidth for each queue.

Tail drop

16

WFQ

Services lowest sequence number (SN). SNs assigned when packet placed into queue, as a function of length and precedence.

Modified tail drop*

4096

CBWFQ

Algorithm is not published. The result of the scheduler provides a percentage guaranteed bandwidth to each queue.

Tail drop or WRED

64

LLQ

Always services low-latency queue first, but each low-latency queue is policed to prevent it from dominating the link.

Tail drop or WRED

64

IP RTP Priority

Always services low-latency queue first, but queue is policed to prevent it from dominating the link.

Tail drop

1

WFQ's modified tail drop includes a per-queue limit, an aggregate limit for all queues, with the ability to dequeue a previously enqueued packet if the new packet has a better SN.

WFQ's modified tail drop includes a per-queue limit, an aggregate limit for all queues, with the ability to dequeue a previously enqueued packet if the new packet has a better SN.

Also make sure and review Tables 4-16 and 4-17 immediately preceding the Foundation Summary.

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