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 prep before the exam, these tables and figures are a convenient way to review the day before the exam.

Table 2-17 lists the IOS classification and marking tools, along with a few key features that differentiate the tools.

Table 2-17. Comparison of Classification and Marking Tools

Tool	Other Functions Besides Class and Mark	Fields That Can Be Examined for Classification	Fields That Can Be Marked[*]
Class-Based marking (CB marking)	None	IP ACLs Any markable fields Input interface MAC addresses All NBAR-enabled fields	IP precedence DSCP 802.1P CoS ISL Priority ATM CLP Frame Relay DE MPLS Experimental QoS Group
Network based application recognition (NBAR)	Statistical information about traffic mix; recognition of applications that use the dynamic port	Extensive list (see Chapter 3, "Classification and Marking")	None; used in conjunction with CB marking

^[*] All Claims about features/functions that may be affected by IOS versions assume version 12.2T, unless otherwise noted.

Table 2-18 outlines the key features of IOS queuing methods.

Table 2-18. Comparison of Queuing Tools

Tool	Maximum Number of Queues	Classification Capabilities	Queue Service Algorithm/ End Result of Algorithm
Priority Queuing (PQ)	4	IP ACL* Input interface Fragments	Strict service; always serves higher-priority queue over lower queue.
Custom Queuing (CQ)	16	IP ACL* Input interface Fragments	Serves a configured number of bytes per queue, per round-robin pass through the queues. Result: Rough percentage of the bandwidth given to each queue under load.
Weighted Fair Queuing (WFQ)	4096	Automatic, based on flows. (Flow identified by source/destination address and port numbers, plus protocol type.)	Each flow uses a different queue. Queues with lower volume and higher IP precedence get more service; high volume, low precedence flows get less service.
Class-Based Weighted Fair Queuing (CBWFQ)	64	IP ACL* NBAR Same as CB marking	Service algorithm not published; results in set percentage bandwidth for each queue under load.
Low Latency Queuing	N/A	Same as CBWFQ	LLQ is a variant of CBWFQ, which makes some queues "priority" queues, always getting served next if a packet is waiting in that queue. It also polices traffic.
Modified Deficit Round-Robin (MDRR)	8	IP precedence	Similar to CQ, but each queue gets an exact percentage of bandwidth. Supports LLQ mechanism as well.

Figure 2-19 depicts the typical points in a network where policing and shaping are typically deployed.

Table 2-19 outlines the key features of IOS policing and shaping tools.

Table 2-19. Comparison of Shaping and Policing Tools

Tool	Policer or Shaper	Interfaces Supported	Per Subinterface, and Per VC, Support
Class-Based policing (CB policing; sometimes just called policer)	Policer	All that are supported by Cisco Express Forwarding (CEF)	Per subinterface
Class-Based shaping	Shaper	All that are supported by CEF	Per subinterface
Frame Relay traffic shaping (FRTS)	Shaper	Frame	Per DLCI

Table 2-20 lists the tools, and the various points for comparison, for Congestion Avoidance tools.

Table 2-20. Comparison of Congestion Avoidance Tools

Tool	Can Be Enabled in IOS?	Weights Based on IP Precedence or DSCP?	Doesn't Drop Packets, but Instead Signals Sender to Slow Down
Random Early Detection (RED)	No	No	No
Weighted Random Early Detection (WRED)	Yes	Yes	No
Explicit Congestion Notification (ECN)	Yes	Yes	Yes

Table 2-21 lists the link-efficiency tools and some of the pertinent comparison points.

Table 2-21. Comparison of Link-Efficiency Tools

Tool	Data Links Supported	Types of Packets to Which Tool Can Be Applied
Payload compression	All; recommended on serial links (T/1, E/1, and slower)	All IP packets
Class-Based RTP header compression (cRTP)	All; recommended on serial links (T/1, E/1, and slower)	All packets with IP/UDP/RTP headers
Class-Based TCP header compression	All; recommended on serial links (T/1, E/1, and slower)	All IP packets with TCP headers
Multilink PPP fragmentation and interleaving (MLPPP LFI)	Multilink PPP	All packets larger than a configured length
Frame Relay fragmentation (FRF[*] )	Frame Relay	All packets larger than a configured length (FRF.12) or all non-VoFR frames (FRF.11c)
Link fragmentation and interleaving for Frame Relay and ATM VCs	Frame Relay and ATM	All IP packets

^[*] The Frame Relay Forum is often referred to as FRF; their document names tend to begin with the letters FRF as well. The QoS feature called Frame Relay Fragmentation is also referred to as FRF.

Table 2-22 lists the RFCs that define DiffServ.

Table 2-22. DiffServ RFCs

RFC	Title	Comments
2474	Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers	Contains the details of the 6-bit DSCP field in IP header.
2475	An Architecture for Differentiated Service	This is the core DiffServ conceptual document.
2597	Assured Forwarding PHB Group	Defines a set of 12 DSCP values and a convention for their use.
2598	An Expedited Forwarding PHB	Defines a single DSCP value as a convention for use as a low-latency class.
3260	New Terminology and Clarifications for DiffServ	Clarifies, but does not supercede, existing DiffServ RFCs.

Figure 2-20 puts some of the DiffServ terminology in context.

Figure 2-21 shows two enterprise networks and two ISPs, with examples of several of the DiffServ terms relating to interconnecting networks.

Figure 2-22 shows the fields inside the ToS byte (per RFC 1349) and the DS field (per RFC 2474).

Table 2-23 lists DSCP values useful for QoS tools that only use precedence, and for those that also use DSCP.

Table 2-23. Default and Class Selector DSCP Values

Name of DSCP Class Selector Values Used by IOS	Binary Value	Equivalent Precedence Value (Decimal)
Default	000000	0
CS1	001000	1
CS2	010000	2
CS3	011000	3
CS4	100000	4
CS5	101000	5
CS6	110000	6
CS7	111000	7

The names of the code points in Table 2-14 match parameters found on IOS DiffServ-compliant classification commands. Because an "all-zeros" DSCP called "default" was already defined, there was no need to create a CS0 DSCP name.

Table 2-24 lists the DiffServ AF DSCPs.

Table 2-24. Assured Forwarding DSCP ValuesNames, Binary, and Decimal

	Low Drop Probability Within Class	Medium Drop Probability Within Class	High Drop Probability Within Class
	Name/Decimal/Binary	Name/Decimal/Binary	Name/Decimal/Binary
Class 1	AF11 / 10 / 001010	AF12 / 12 / 001100	AF13 / 14 / 001110
Class 2	AF21 / 18 / 010010	AF22 / 20 / 010100	AF23 / 22 / 010110
Class 3	AF31 / 26 / 011010	AF32 / 28 / 011100	AF33 / 30 / 011110
Class 4	AF41 / 34 / 100010	AF42 / 36 / 100100	AF43 / 38 / 100110

Table 2-25 summarizes many of the key points about the various DiffServ PHBs.

Table 2-25. Comparison of DiffServ PHBs

PHB	Key Components	Names of DSCPs
Best effort (BE)	PHB for getting no specific QoS treatment	DSCP BE (default)
Class selector (CS)	Uses 8 DSCPs, all with binary 0s for the last 3 bits. Used for backward compatibility with IP precedence. Uses "bigger-is-better" logicthe bigger the DSCP, the better the QoS treatment.	CS1, CS2, CS3, CS4, CS5, CS6, CS7
Assured forwarding (AF)	PHB consists of 2 components: queuing to provide a minimum bandwidth to each for 4 different queues, and 3 drop thresholds inside each queue. DSCPs do not always follow the "bigger-is-better" logic.	AF11, AF12, AF13, AF21, AF22, AF23, AF31, AF32, AF33, AF41, AF42, AF43
Expedited forwarding (EF)	PHB also has 2 components: queuing to provide low delay/jitter/loss and a guaranteed amount of bandwidth, and policing to prevent EF from preventing other types of traffic from getting enough bandwidth.	EF