End users should not experience delays
in computing responsiveness as the internetwork grows. In addition,
as an administrator, you need to be aware of the latency issues that
are unique to each protocol operating in the internetwork. Latency
is the delay experienced by traffic as it crosses a network. Some
protocols may time out when the latency is too great. Routers are
responsive when they can accommodate the latency needs of each
protocol without affecting response time at the desktop.
Path Optimization
One of the primary advantages of a router is
its capability to help you implement a logical environment in which
optimal paths for traffic are automatically selected. Routers rely
on routing protocols that are associated with the various
network-layer protocols to accomplish this automated path
optimization.
Depending on the network protocols implemented, routers permit
you to implement routing environments that suit your specific
requirements. For example, in an IP internetwork, Cisco routers can
support all widely implemented routing protocols, including OSPF,
Routing Information Protocol (RIP), IGRP, Border Gateway Protocol (BGP),
and Exterior Gateway Protocol (EGP). Key built-in
capabilities that promote path optimization include rapid and
controllable route convergence and tunable routing metrics and
timers.
Convergence is the process of agreement, by all routers, on
optimal routes. When a network event causes routes to either halt
operation or become available, routers distribute routing update
messages. Routing update messages permeate networks, stimulating
recalculation of optimal routes and eventually causing all routers
to agree on these routes. Routing algorithms that converge slowly
can cause routing loops or network outages.
Many different metrics are used in routing algorithms. Some
sophisticated routing algorithms base route selection on a
combination of multiple metrics, resulting in the calculation of a
single hybrid metric. IGRP uses one of the most sophisticated
distance vector routing algorithms. It combines values for
bandwidth, load, and delay to create a composite metric value.
Link-state routing protocols, such as OSPF and Intermediate
System-to-Intermediate System (IS-IS), employ a metric that
represents the cost associated with a given path.
Traffic Prioritization
Although some network protocols can prioritize internal
homogeneous traffic, the router prioritizes the heterogeneous
traffic flows. Such traffic prioritization enables policy-based
routing and ensures that protocols carrying mission-critical data
take precedence over less important traffic.
The main Cisco IOS feature that supports responsiveness on slow
wide-area links is queuing. Queuing is the reordering of traffic
packets after their arrival and dispatching them in the new order,
favoring desirable traffic. Cisco supports three forms of queuing:
Weighted Fair Queuing (WFQ), which is on by default on slow WAN
links, and priority and custom queuing, which can be manually
optimized. Cisco IOS features that support responsiveness follow.
Priority Queuing
Priority queuing allows the network administrator to prioritize
traffic. When a particular traffic type is prioritized higher than
all other traffic types, it is allowed to go through before them. In
this way, the priority traffic is assured of getting through, but
other types of traffic may not get through in a timely manner.
Traffic can be classified according to various criteria, including
protocol and subprotocol type, and then queued on one of four output
queues (high, medium, normal, or low priority). For IP traffic,
additional fine-tuning is possible. Priority queuing is most useful
on low-speed serial links. Priority queuing can be used to segregate
traffic by priority level, speeding the transit of certain packets
through the network. In the graphic, for example, the router
prioritizes voice and video traffic over data traffic.
Custom Queuing
Priority queuing introduces a fairness problem in that packets
classified to lower-priority queues might not get serviced in a
timely manner, or at all. Custom queuing is designed to address this
problem. Custom queuing allows more granularity than priority
queuing. In fact, this feature is commonly used in the
internetworking environment in which multiple higher-layer protocols
are supported. Custom queuing reserves bandwidth for a specific
protocol, thus allowing mission-critical traffic to receive a
guaranteed minimum amount of bandwidth at any time. The intent is to
reserve bandwidth for a particular type of traffic. Each traffic
type gets a share of the available bandwidth; thus certain types of
traffic can be allocated larger or smaller amounts of bandwidth,
depending on such parameters as sensitivity to latency.
Custom queuing prioritizes multiprotocol traffic. A maximum of 16
queues can be built with custom queuing. Each queue is serviced
sequentially until the number of bytes sent exceeds the configurable
byte count or the queue is empty. Custom queuing is designed for
environments that want to ensure a minimum level of service for all
protocols. In today's multiprotocol internetwork environment, this
important feature allows protocols of different characteristics to
share the media.
Weighted Fair Queuing
WFQ is an automated method that provides fair bandwidth
allocation to all network traffic. It ensures that high-bandwidth
conversations do not consume all bandwidth. WFQ is a traffic
priority management algorithm that uses the time-division
multiplexing (TDM) model to divide the available bandwidth among
clients that share the same interface. In TDM, each client is
allocated a time slice in a round-robin fashion. In WFQ, the
bandwidth is distributed evenly among clients so that each client
gets a fair share if everyone has the same weighting.
You can assign a different set of weights, for example, through
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