1.2 Key Characteristics of Scalable Internetworks
1.2.3 Making the network responsive
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 the type --- of-service (ToS) feature, so that more bandwidth is allocated. If every client is allocated the same bandwidth independent of the arrival rates, the low-volume traffic has effective priority over high-volume traffic. The use of weighting allows time-delay-sensitive traffic to obtain additional bandwidth, thus guaranteeing consistent response time under heavy traffic conditions. WFQ and custom queuing require the installation of access lists; the bandwidth has to be preallocated and priorities have to be predefined. This is clearly a burden. Sometimes, network administrators cannot identify and prioritize network traffic in real time. WFQ sorts among individual traffic streams without the administrative burden associated with the other two types of queuing.