31. The class-mark policy map is applied
to the input interface, Ethernet 0/0. On the output
interface, three class maps have been created: voice-out,
videoconferencing-out, and interactive-out. The voice-out class
map will match the DSCP field EF. The videoconferencing-out
class map will match the DSCP field AF 41. The interactive-out
class map will match the DSCP field AF 31. As shown in the
figure, the qos-policy policy map will then do the following:
- If the packet matches the voice-out class map, the
packet priority will be set to 10 percent of the bandwidth. If
the packet matches the videoconferencing-out class map, the
packet priority will be set to 20 percent of the bandwidth. If
the packet matches the interactive-out class map, the packet
priority will be set to 30 percent of the bandwidth. All other
packets will be classified as class-default and fair queuing
will be performed on them.
- The class-mark policy map
is applied to the output interface, serial 0/0.
Content 4.3 Introducing Queuing
Implementations 4.3.1 Congestion and
Queuing Congestion can occur anywhere within a network
where speed mismatches (for example, a 1000-Mbps link feeding a
100-Mbps link), aggregation (for example, multiple 100-Mbps
links feeding an upstream 100-Mbps link), or confluence (the
joining of two or more traffic streams) accrue. Figure
illustrates the concept.Congestion-management features control
the congestion when it occurs. One way that network elements
handle an overflow of arriving traffic is to use a queuing
algorithm to sort the traffic and then determine some method of
prioritizing it onto an output link. Each queuing algorithm was
designed to solve a specific network traffic problem and has a
particular effect on network performance. Many algorithms have
been designed to serve different needs. A well-designed queuing
algorithm provides some bandwidth and delay guarantees to
priority traffic. Example: Congestion Caused by Speed
Mismatch
Speed mismatches, as shown in Figure are the
most typical cause of congestion in a network. Speed mismatches
are the most common reason for congestion. It is possible to
have persistent congestion when traffic is moving from a LAN to
a WAN, such as when traffic moves from a high-speed LAN
environment (100 or 1000 Mbps) to lower-speed WAN links (1 or 2
Mbps). Speed mismatches are also common in LAN-to-LAN
environments when, for example, a 1000-Mbps link feeds into a
100-Mbps link, but in those cases they are transient.
Example: Congestion Caused by Aggregation
The second
most common source of congestion is points of aggregation in a
network as shown in Figure . Typical points of aggregation
occur in WANs when multiple remote sites feed into a central
site. In a LAN environment, congestion resulting from
aggregation often occurs at the distribution layer of networks
where the access layer devices feed traffic to the distribution
layer switches.
Content 4.3 Introducing
Queuing Implementations 4.3.2 Congestion
Management - Queuing Algorithms Queuing is designed to
accommodate temporary congestion on an interface of a network
device by storing excess packets in buffers until bandwidth
becomes available or until the queue depth is exhausted and
packets have to be dropped. Queuing is a congestion-management
mechanism that allows you to control congestion by determining
the order in which identified packets leave an interface based
on priorities assigned to those packets. Congestion management
entails creating queues, assigning packets to those queues
based on the classification of the packet, and scheduling the
packets in a queue for transmission. Cisco IOS routers support
several queuing methods to meet the varying bandwidth, jitter,
and delay requirements of different applications. The default
mechanism on most interfaces is the very simplistic first-in,
first-out (FIFO) queue. Some traffic types, such as voice and
video, have very demanding delay and jitter requirements, so
more sophisticated queuing mechanisms must be configured on
interfaces used by voice and video traffic. Figure defines
queuing. Congestion and Queuing
Complex queuing
generally happens on outbound interfaces only. A router queues
packets it sends out an interface. During periods with low
traffic loads, when no congestion occurs, packets leave the
interface as soon as they arrive. During periods of transmit
congestion at the outgoing interface, packets arrive faster
than the interface can send them. When you use
congestion-management features, packets accumulating at an
interface are placed in software queues according to their
assigned priority and the queuing mechanism configured for the
interface. They are then scheduled for transmission when the
hardware buffer of the interface is free to send them. Figure
illustrates the process. The router determines the order of
packet transmission by controlling which packets go into each
queue and how the queues are serviced with respect to each
other. Queuing Algorithm Introduction
Figure lists
some of the key queuing algorithms: - FIFO:
First-in, first-out; the simplest algorithm
- Priority queuing (PQ): Allows traffic to be
prioritized
- Round robin: Allows several queues
to share bandwidth
- Weighted round robin (WRR):
Allows sharing of bandwidth with prioritization
Content 4.3 Introducing Queuing
Implementations 4.3.3 FIFO FIFO is the
simplest queuing algorithm. FIFO provides basic
store-and-forward capability as shown in Figure . FIFO is the
default queuing algorithm in some instances, thus requiring no
configuration. In its simplest form, FIFO queuing—also known as
first-come, first-served queuing—involves storing packets when
the network is congested and forwarding them in order of
arrival when the network is no longer congested. FIFO embodies
no concept of priority or classes of traffic and consequently
makes no decision about packet priority. There is only one
queue, and all packets are treated equally. Packets are placed
into a single queue and transmitted in the order in which they
arrive. Higher-priority packets are not transmitted faster than
lower-priority packets. When FIFO is used, ill-behaved sources
can consume all the bandwidth, and bursty sources can cause
delays to time-sensitive or important traffic; also, important
traffic can be dropped because less important traffic has
filled the queue. When no other queuing strategies are
configured, all interfaces except serial interfaces at E1
(2.048 Mbps) and below use FIFO by default. FIFO, which is the
fastest method of queuing, is effective for links that have
little delay and minimal congestion. If your link has very
little congestion, FIFO queuing may be the only queuing you
need to use. All individual queues are, in fact, FIFO queues.
Other queuing methods rely on FIFO as the underlying queuing
mechanism for the discrete queues within more complex queuing
strategies that support advanced functions such as
prioritization. Note
Serial interfaces at E1 (2.048
Mbps) and below use weighted fair queuing (WFQ) by default.
Content 4.3 Introducing Queuing
Implementations 4.3.4 Priority Queuing
Figure shows PQ that allows you to prioritize
traffic in the network. You configure four traffic priorities.
You can define a series of filters based on packet
characteristics to cause the router to place traffic into these
four queues; the queue with the highest priority is serviced
first until it is empty, then the lower queues are serviced in
sequence. During transmission, PQ gives priority queues
absolute preferential treatment over low-priority queues;
important traffic, given the highest priority, will always take
precedence over less important traffic. Packets are classified
based on user-specified criteria and placed in one of the four
output queues—one, two, three, and four—based on the assigned