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QoS 配置基础
Configuration of QoS
In last month's newsletter I discussed the mechanisms QoS provides for
delivering traffic with different priorities and assurances. This month
we are going to look at the specific configurations for enabling the
QoS assurances. Configuration of QoS is somewhat confusing as there are
many types of QoS "knobs" that can be configured on a router or a
switch. Hopefully this newsletter will provide the necessary
instruction for getting through some of the complexities of QoS
configurations. This newsletter focuses on the QoS configurations
relevant to Cisco IOS not CatOS. Layer2 QoS is another topic entirely
and will be reserved for future newsletters.
To refresh our memory from last month, let's review QoS briefly. QoS is
a set of tools, or "knobs" as they are sometimes called, that are
configured on network elements (usually routers and switches) to
enforce traffic delivery policies for certain types of traffic. QoS can
be broken down into three levels or models to provide end-to-end levels
of service to traffic delivered over the network infrastructure. These
models are called best-effort service, integrated service and
differentiated service. Within these models, QoS defines different
techniques to handle traffic. These techniques are congestion
avoidance, congestion management, policing and traffic shaping. We will
focus on the configuration of the congestion management techniques in
this newsletter. Congestion avoidance, policing and traffic shaping
techniques will be discussed next month.
Congestion Management
Congestion management techniques control the prioritization of traffic
as it leaves the queues, it does not minimize congestion. Cisco
provides several different techniques for providing congestion
management. The mechanisms include First In First Out Queuing (FIFO),
Weighted Fair Queuing (WFQ), Priority Queuing and Custom Queuing. In
addition, there is Class Based Weighted Fair Queuing (CBWFQ), IP RTP
Priority (or Priority Queue WFQ) and Low Latency Queuing (LLC).
First in First Out Queuing (FIFO)
FIFO has no concept of priority or classes of traffic and does not
provide any prioritization of packets
Weighted Fair Queueing (WFQ)
WFQ provides allocation of bandwidth to network conversations. Weights
are assigned based on source and destination patterns. WFQ
automatically gives precedence for low-volume traffic such as telnet
and HTTP over high volume traffic such as FTP. WFQ is enabled by
default on interfaces that run below E1 speeds (2.048 Mbps).
Priority Queuing (PQ)
PQ allows you to define four queues for traffic prioritization. Traffic
is matched using match criteria such as IP address, port numbers and
interfaces. The four priorities are defined as high, medium, normal and
low. The queues are served in a preferential manner. The router will
not service the lower-priority queues until all packets from the
higher-priority queues have been delivered. This can be detrimental if
not configured properly as all of the bandwidth can be consumed by the
higher-priority queues, not allowing the router to transmit data out of
the other queues. Let's say you want to do the following for traffic
being forwarded out interface serial 0.
Telnet (TCP port 23) traffic is assigned to the high-priority queue.
AppleTalk and IPX are given the medium-priority queue.
