102 Chapter 4: Per-Hop Behavior: Resource Allocation I
Frequently Asked Questions
Q — Explain the queuing mechanisms using the frequently used airline industry
analogy.
A — Comparing the queuing analogy to the airline industry, a FIFO queue is analogous
to an airline queuing model whereby passengers belonging to all classes—first,
business, and economy—have a single queue to board the plane. No service
differentiation exists in this queuing model.
CBWFQ, MWRR, and MDRR are analogous to an airline queuing service that
assigns a separate queue for each class of passengers, with a weight or bandwidth
assigned to each queue. A passenger queue is serviced at a rate determined by its
weight or bandwidth allocation.
In an airline model analogous to priority queuing, a separate queue for first,
business, and economy is used, and they are served in strict priority—in other
words, those in the first-class queue are served first. An airline model serving ten
passengers from first class, six from business class, and four from economy class
on a round-robin basis is similar to the custom queuing model.
Q — How is the bandwidth used in CBWFQ when one class is not using its allocated
bandwidth?
A — The bandwidth specified for a traffic class in CBWFQ is its minimum guaranteed
bandwidth at times of congestion. If a traffic class is not using its allocated
bandwidth to its fullest, the other traffic classes in the queuing system can use any
leftover bandwidth in proportion to their assigned bandwidth.
Q — Are there any exceptions to the precedence-based weight assignment procedure in
flow-based WFQ?
A — Yes. Weight is based on IP precedence for IP traffic unless any of the following
four conditions applies:
RSVP has negotiated a specific weight for a specific flow.
The traffic is voice and Local Frame Interleave is on.
The traffic is a locally generated packet, such as a routing update packet set
with an internal packet priority flag.
A strict priority queue for voice is enabled by using the ip rtp priority
command.
In each of those cases, a weight specific to the application is used.
Q — How does flow-based WFQ treat non-IP traffic?
A — There are separate classification routines for non-IP flows based on the packet’s
data link layer type (for example, IPX, AppleTalk, DECnet, BRIDGE, RSRB). All

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