Queuing Tools 299
Summary of Queuing Tool Features
The classification feature and the scheduler are the two most important features of a queuing tool. The drop policy, and the maximum number of queues supported, is also very important. Although this chapter covers these features in detail, a brief summary may be useful to avoid losing some of the details.
Table 4-16 lists the queuing tools covered in detail in this chapter, cross-referenced with the classification fields that each supports.
Table 4-16 Classification Fields Used by Classification and Marking Tools
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RTP |
Classification Field* |
WFQ |
CBWFQ |
LLQ |
Priority |
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Extended IP ACLs |
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X |
X |
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Incoming interface |
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X |
X |
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Packet length |
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Fragment |
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TCP/UDP port numbers |
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X |
X |
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Flow (S/D address and port, plus protocol |
X |
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ToS byte |
X** |
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QoS group |
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X |
X |
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Even-numbered UDP ports, in a prespecified |
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X |
X |
X |
range |
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IP Precedence |
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X |
X |
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IP DSCP |
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X |
X |
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CoS |
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X |
X |
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NBAR |
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X |
X |
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MPLS Experimental bits |
X |
X |
X |
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Input interface |
X |
X |
X |
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*Some Fields can be matched via ACL as well as being matched directly. For instance, DSCP can be matched with an ACL. In the table, if an X is listed, it means the tool can directly match that field.
**As listed in the chapter, some documents claim that WFQ also classifies based on the ToS byte.
Table 4-17 lists the tools covered in this chapter, along with a brief synopsis of the scheduler, drop, and maximum queues supported.
300 Chapter 4: Congestion Management
Table 4-17 Summary of Scheduler, Drop, and Number of Queues
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Max # of |
Tool |
Scheduler |
Drop Policy |
Queues |
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FIFO |
Services packets in the same order that they |
Tail drop |
1 |
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arrived. |
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PQ |
Always services higher-priority queues first; |
Tail drop |
4 |
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the result is great service for the High queue, |
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with potential for 100% of link bandwidth. |
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Service degrades quickly for lower-priority |
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queues. |
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CQ |
Services packets from a queue until a byte |
Tail drop |
16 |
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count is reached; round-robins through the |
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queues, servicing the different byte counts for |
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each queue. The effect is to reserve a |
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percentage of link bandwidth for each queue. |
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WFQ |
Services lowest sequence number (SN). SNs |
Modified tail drop* |
4096 |
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assigned when packet placed into queue, as a |
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function of length and precedence. |
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CBWFQ |
Algorithm is not published. The result of the |
Tail drop or WRED |
64 |
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scheduler provides a percentage guaranteed |
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bandwidth to each queue. |
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LLQ |
Always services low-latency queue first, but |
Tail drop or WRED |
64 |
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each low-latency queue is policed to prevent it |
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from dominating the link. |
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IP RTP Priority |
Always services low-latency queue first, but |
Tail drop |
1 |
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queue is policed to prevent it from dominating |
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the link. |
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*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.
Certainly, one of the difficulties in passing any QoS exam is memorizing some of the details that, frankly, you would never need to memorize to do your job well. Take the time to read the “Foundation Summary,” review the example configurations scattered throughout this book, and focus on tables such as the two in this summary section to memorize the details long enough to pass the exam!