11 Explain why the first packet in a flow may not retain the proper CoS values when a Catalyst 6500 is configured with a PFC.
If this flow is not in the flow cache of the PFC or the CEF FIB of the PFC2, the first packet of the flow is forwarded to the MSFC for a routing decision. After a routing decision has been made, the fist packet in the flow is routed to the correct interface.
This rewrites the CoS value to 0. The routing decision populates the flow cache or the CEF FIB, causing subsequent packets to be switched and thereby retain the original CoS value.
12 What is the difference between Hybrid mode and Native mode?
Hybrid mode refers to the Catalyst operating system, whereas Native mode refers to an IOS operating system on the Catalyst 6500 series switches.
13 What command is used on a Catalyst 6500 running in Hybrid mode to place ports 2/1 through 2/10 in VLAN 10?
set port auxiliaryvlan 2/1-10 10
14 What command enables QoS on a 6500 running in Native mode?
mls qos
15 What queue and threshold is recommended for call control traffic on a Catalyst 6500?
2q1t
16 Which series of Ethernet line cards are preferred in the Catalyst 6500 series for QoS?
Why?
The 65xx series of Ethernet line cards are preferred due to the increase in buffer size and the addition of a priority queue.
Catalyst 4500/4000 Series of Switches
17 Which supervisor engine is preferred on the Catalyst 4500/4000 series? Why?
The Supervisor Engine III or the Supervisor Engine IV are preferred because of the addition of a priority queue, three standard queues, and the ability to use QoS access lists.
18 In which queue number does the priority queue reside on a Catalyst 4500 or 4000 with a Supervisor II Engine?
There is no priority queue on a Catalyst 4500 or 4000 with a Supervisor II Engine.
19 In which queue number does the priority queue reside on a Catalyst 4500 or 4000 with a Supervisor III Engine?
The priority queue in a Catalyst 4500 or 4000 with a Supervisor III Engine resides in Queue 3.
20 What keyword(s) is used to enable the priority queue on a Catalyst 4500 or 4000 with a Supervisor III module?
tx-queue 3 priority high
21 Name the four methods a Catalyst 4500 with a Supervisor III uses to classify traffic.
CoS values
IP precedence values IP DSCP values IP ACLs
22 What command enables you to rewrite a the CoS value of a PC attached to an IP Phone with Catalyst IOS switches?
switchport priority extend cos
23 What trust state are the ports if a 4500 with a Supervisor III in when QoS is enabled?
Untrusted.
24 What is dynamic buffer limiting?
DBL is an automated algorithm that manages misbehaving traffic flows by tracking the buffering for each traffic flow and limiting the flow if excessive demands are placed on a buffer.
Catalyst 3550/3524 Series of Switches
25 In which queue number does the priority queue reside on a Catalyst 3550?
The priority queue in a Catalyst 3550 resides in Queue 4.
26 What command enables QoS on the Catalyst 3550?
mls qos
27 What trust state are the ports of a 3550 in when QoS is enabled?
Untrusted.
28 Name the four methods a Catalyst 3550 can use to classify traffic.
CoS values
IP precedence values IP DSCP values IP ACLs
29 What command enables QoS on the Catalyst 3524?
QoS is enabled by default in a Catalyst 3524. No command is needed.
30 When is a GigaStack GBIC module an acceptable QoS choice for cascading Catalyst 3524 switches together?
The GigaStack modules are acceptable for QoS under either of the two following conditions:
— The Catalyst 3524 switches are cascaded using both ports of the GigaStack modules causing half-duplex connections; however, no real-time
applications are present on the network.
— The Catalyst 3524 switches are cascaded using only a single port of the GigaStack modules, resulting in full-duplex connections. Real-time applications can be present on the network.
A P P E N D I X B
Topics on the CCIP QoS Exam
The CCIP QoS exam covers some topics that are not covered on the DQOS exam. As stated in the Introduction to this book, the core chapters of this book focus on the more widely popular DQOS exam. However, some of you may be studying for the less-popular QoS exam, so this appendix covers many of the topics on the QoS exam, but not on the DQOS exam.
Keep in mind that, when the exams change, you should check both www.cisco.com and www.ciscopress.com for more information. The Cisco website is the main authoritative source for information about all Cisco exams. Additionally, the next time these exams change, the authors of this book will post information at www.ciscopress.com/1587200589 about the changes. Included there will be information about the specific changes to the exams, and how best to use your book to pass the revised exam(s).
This appendix includes two sections. One covers a classification and marking tool, and the other section covers several queuing tools. Each section includes explanations and config- uration examples as appropriate. Questions are included at the end of each section, not at the end of the appendix, so that you can consider questions for each general topic area. The answers are included in the chapter for your convenience.
832 Appendix B: Topics on the CCIP QoS Exam: Topics on the CCIP QoS Exam
Foundation Topics
For those of you studying for both exams, you could begin by studying for, and passing, the DQOS exam. To study, you could just attack each chapter of this book in succession, and ignore this appendix. After you have passed the DQOS exam, you can then study using this appendix, and pick up the additional information you need.
For those of you just studying for the CCIP QoS exam, not only should you plan to study the topics in this chapter, you can also ignore some topics in the core chapters of this book. The Introduction to this book suggests some options for your study plan when studying just for the QoS exam. Table B-1 summarizes the major topics from Chapters 1 through 10 that you can ignore if you are preparing just for the QoS exam.
Any time you study for a Cisco exam, you should always pull a list of exam topics from www.cisco.com for the exam. Use these exam topics as your primary guide to know what to study for the exam; after all, the exam questions typically test whether you can do what the exam topic describes. As mentioned in the Introduction to this book, you can also check www.ciscopress.com/1587200589, where the authors of this book will post hints and tips when exam changes occur.
Table B-1 Preparing for the QOS Exam
Book
Chapter Suggested Deviation as Compared with DQOS Study Plan 1 None; read and study the entire chapter.
2 None; read and study the entire chapter.
3 Read and study all of Chapter 3; then come to this appendix and read the section “QoS Policy Propagation with BGP (QPPB).” QPPB is the one classification and marking tool that is on the QoS exam, but not on the DQOS exam.
4 Read and study Chapter 3 entirely; then come to this appendix and read the section
“Congestion Management (Queuing).” This section covers several queuing tools not covered on the DQOS exam.
5 None; read and study the entire chapter.
6 None; read and study the entire chapter.
7 None; read and study the entire chapter.
8 Ignore all topics covering voice CAC, but do read all coverage of RSVP.
9 You do not need to read this chapter; the topics are not covered on the QoS exam.
10 You do not need to read this chapter; the topics are not covered on the QoS exam.
Classification and Marking 833
Classification and Marking
Chapter 3, “Classification and Marking,” covers classification and marking tools. QoS policy propagation with BGP (QPPB) is the one tool in this category that is on the QoS exam, but not on the DQOS exam.
QoS Policy Propagation with BGP (QPPB)
QoS policies that differentiate between different types of traffic can be most easily defined for a single enterprise network. For instance, one enterprise may want to treat important web traffic, not-important web traffic, and all other data traffic as three different classes, and use different classes for voice and video traffic. For the Internet, however, a single QoS policy would never work. Differentiated services (DiffServ), which was designed specifically to address QoS over the Internet, defines the role of ingress boundary nodes to re-mark traffic as it enters a different DiffServ domain, essentially changing the differentiated services code point (DSCP) to reflect the QoS policies of each respective DiffServ domain. This practice allows each DiffServ domain to set its own QoS policies.
QoS policies that classify traffic based on the characteristics of the flow—voice, video, different data applications, and so on—can be defined and used in enterprises and by service providers.
Enterprises can afford to be more selective, because a single group can often set the QoS poli- cies. For instance, an enterprise could classify based on the IP addresses of some mission- critical servers. QoS policies for Internet service providers (ISPs) tend to be less specific than those for an enterprise, because ISPs have many customers. However, ISPs can still implement QoS policies based on the type of traffic contained in the packet.
ISPs may want a QoS policy just to prefer one customer’s traffic over another. In Figure B-1, for instance, consider ISP 1, which has two customers. Customer 1 has agreed to pay a premium for its Internet service, in return for ISP 1 agreeing to provide better latency and delay charac- teristics for the traffic. Customer 2 keeps paying the same amount as always, and still gets best- effort service.
The QoS tools only need to differentiate between Customer 1 and Customer 2 traffic to support this policy. So, for packets flowing from right to left, if the source IP address is an IP address in Customer 1’s network, the packet might be marked with precedence 4, for instance. Similar- ly, when packets flow left to right, these same tools could examine the destination IP address, and if it’s part of Customer 1’s network, precedence 4 could be marked. Packets to or from Cus- tomer 2 could be marked with precedence 0.
834 Appendix B: Topics on the CCIP QoS Exam
Figure B-1 QoS Policy Based on Customer—Customer 1 and Customer 2
Class-based (CB) marking, policy-based routing (PBR), and committed access rate (CAR) could perform the necessary marking to support premium and best-effort customer services.
However, each of these three tools has some negative side effects. For all three tools, that clas- sification would require an IP ACL for matching the packets, for all packets. For an ISP with many customers, however, classifying and marking packets based on referencing ACLs for a large number of packets may induce too much overhead traffic. Suppose further that ISP 1 and ISP 2 agree to support each other’s premium and best-effort customers in a similar manner. The two ISP’s would have to continually exchange information about which networks are premium, and which are not, if they are using IP ACLs to classify the traffic. Additionally, when new cus- tomers are added, ISP 1 may be waiting on ISP 2 to update their QoS configuration before the desired level of service is offered to the new customer.
To overcome the two issues—the scalability of classifying based on ACLs, and the administra- tive problems of just listing the networks that need premium services—QPPB was created.
QPPB allows marking of packets based on an IP precedence or QoS group value associated with a Border Gateway Protocol (BGP) route. For instance, the BGP route for Customer 1’s net- work, Network A, could be given a BGP path attribute that both ISP 1 and ISP 2 agree should mean that this network receives better QoS service. Because BGP already advertises the routes, and the QoS policy is based on the networks described in the routes, QPPB marking can be done more efficiently than with the other classification and marking tools.
Figure B-2 shows the basic process in action. In this example, R3 is configured to use QPPB, although it would likely be used in several places around the network.
QPPB follows two steps: marking routes, and then marking packets based on the values marked on the routing entries. BGP routing information includes the network numbers used by the var- ious customers, and other BGP path attributes. Because Cisco has worked hard over the years to streamline the process of table lookup in the routing table, to reduce per-packet processing for the forwarding process, QPPB can use this same efficient table-lookup process to reduce classification and marking overhead.
Customer 3 Customer 1 —
Network 1 ISP1
ISP2
R1 R2 R3 R4
AS 200 AS 300 AS 400
AS 100
Customer 2 — Network 2
AS 500
Classification and Marking 835
Figure B-2 QPPB—Basic Components
For reference, Tables B-2 and B-3 summarize the QPPB configuration and exec commands, respectively.
Table B-2 Configuration Command Reference for QPPB
Command Mode and Function
route-mapmap-tag [permit | deny] [sequence-number]
Global command; creates a route map entry match ip address {access-list-number |
access-list-name} [... access-list-number | ...
access-list-name]
Route-map subcommand; used to match IP packets based on parameters matchable with an IP ACL match length minimum-length maximum-
length
Route-map subcommand; used to mach IP packets based on their length
set ip precedence number | name Route-map subcommand; sets IP precedence vale using the decimal number of name.
continues
Customer 3 Customer 1 —
Network 1 ISP1
ISP2
R1 R2 R3 R4
AS 200 AS 300 AS 400
AS 100
Customer 2 — Network 2
AS 500 Step 1: BGP Routing Table
Classify:
Check BGP learned routes against a route-map Mark:
IP Precedence and QoS Group of matched route, if any
Step 2: Classify Based on Route:
Check the source IP address of packet versus routing table
Mark Packet:
IP Precedence or QoS Group of matched route, if any
Step 2: Classify Based on Route:
Check the destination IP address of packet versus routing table
Mark Packet:
IP Precedence or QoS Group of matched route, if any
836 Appendix B: Topics on the CCIP QoS Exam
QPPB can be a confusing topic. The rest of this section discusses more detail about how QPPB works and how to configure it. One key to understanding QPPB, in spite of some of the detail, is to keep these two key points in mind as you read the following sections:
• QPPB classifies BGP routes based on the BGP routes’ attributes, and marks BPG routes with an IP precedence or QoS group value.
• QPPB classifies packets based on the associated routing table entries, and marks the packets based on the marked values in the routing table entry.
set ip qos-group group-id Route-map subcommand; sets a group ID in the routing table for classification throughout the network.
table-mapmap-name BGP subcommand; used to modify values related to BGP learned routes, including precedence and QoS group
ip community-listcommunity-list-number {permit | deny} community-number
Global command; used to create a community list, which matches values in the BGP community string ip as-path access-listaccess-list-number
{permit | deny} as-regexp
Global command; used to create an autonomous system (AS) path list, which matches values in the autonomous system number (ASN) path BGP attribute ip bgp-community new-format BGP subcommand; used to make IOS use the AA:NN
format for community values, with AA being the ASN, and NN being a user-defined value
bgp-policy ip-prec-map Interface subcommand; enables QPPB for packets entering the interface, marking IP precedence bgp-policy ip-qos-map Interface subcommand; enables QPPB for packets
entering the interface, marking QoS group Table B-3 Exec Command Reference for QPPB
Command Function
show ip bgp Shows BGP routing table
show ip routeprefix Shows IP routing table entries, including precedence values
show ip bgp community-listcommunity-list- number
Lists configuration of the community list show ip cefnetwork Shows the Cisco Express Forwarding (CEF)
Forwarding Information Base (FIB), including the marked QoS values
Table B-2 Configuration Command Reference for QPPB (Continued)
Command Mode and Function
Classification and Marking 837
NOTE Because QPPB involves quite a few detailed concepts and configuration, some of the true details of how QPPB works are glossed over during the initial discussions. These details are explained at the end of this section in the subsection titled “QPPB: The Hidden Details.”
QPPB Route Marking: Step 1
QPPB allows routers to mark packets based on information contained in the routing table.
Before packets can be marked, QPPB first must somehow associate a particular marked valued with a particular route. QPPB, as the name implies, accomplishes this task using BGP. This first step can almost be considered as a separate classification and marking step by itself, because BGP routes are classified, based on information that describes the route, and marked with some QoS value.
The classification feature of QPPB can examine many of the BGP path attributes. The two most useful BGP attributes for QPPB are the autonomous system number (ASN) sequence, referred to as the autonomous system path, and the community string. The autonomous system path contains the ordered list of ASNs, representing the ASNs between a router and the autono- mous system of the network described in the route. In Figure B-2, R1 receives a BGP update for Network 1, listing ASNs 300 and 400 in the autonomous system path and a BGP update for Network 2, listing ASNs 300 and 500 in the autonomous system path. QPPB can be used to mark the route to Network 1 (Customer 1) with one precedence value, while marking the route to Network 2 (Customer 2) with another precedence value, based on the autonomous system path received for the route to each customer.
The community attribute provides a little more control than does the autonomous system path.
The autonomous system path is used to avoid routing loops, and the contents of the autonomous system path changes when aggregate routes are formed. The community attribute, however, allows the engineer to essentially mark any valid value. For instance, R3 could set the commu- nity attribute to 10:200 for the route to Network 1, and advertise that route toward the left side of the network diagram. Other routers could then use QPPB to classify based on the community attribute of 10:200, and assign the appropriate precedence value to the route to Network 1.
QPPB configuration would essentially create logic as follows: “If the community attribute contains 10:200, mark the route with precedence 4.”
Example B-1 lists the QPPB configuration just for marking the route based on the autonomous system number. With this configuration, no packets are marked, because the QPPB configura- tion is not complete. (The complete configuration appears in the next section.) QPPB is a two- step process, and Example B-1 just shows the configuration for the first step.