Enterprise QoS Solution Reference Network Design Guide

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WAN Edge QoS Design Considerations 3-2 Figure 3-1 Where is QoS Needed over the WAN? WAN Edge QoS Design Considerations Software QoS Campus Distribution Switches LAN Edges WAN Aggregator Enterprise QoS Solution Reference Network Design Guide Queuing/Dropping/Shaping/ Link-Efficiency Policies for Campus-to-Branch Traffic Chapter 3 WAN Aggregator QoS Design QoS policies required on WAN aggregators include queuing, shaping, selective dropping, and link-efficiency policies in the outbound direction of the WAN link. Traffic is assumed to be correctly classified and marked (at Layer 3) before WAN aggregator ingress. Remember, Layer 3 markings (preferably DSCP) are media independent and traverse the WAN media, whereas Layer 2 CoS is lost when the media switches from Ethernet to WAN media. Several factors must be kept in mind when designing and deploying QoS polices on WAN edges. Some of these considerations were introduced in earlier chapters. They are re-emphasized here to underscore their importance to the context of the WAN QoS designs that follow. Unlike LAN (Catalyst) queuing, which is done in hardware, WAN edge QoS is performed within Cisco IOS Software. If the WAN aggregator is homing several hundred remote branches, the collective CPU required to administer complex QoS policies might be more than some older devices can provide. The main point to keep in mind is that QoS entails a marginal CPU load. WAN topologies and QoS policies should be designed to limit the average CPU utilization of the WAN aggregator to 75 percent (or lower) because this leaves cycles available to respond efficiently to routing updates. Bandwidth Provisioning for Best-Effort Traffic WAN Edges WAN As discussed previously, the Best-Effort class is the default class for all data traffic. Only if an application has been selected for preferential or deferential treatment is it removed from the default class. Because many enterprises have several hundreds, if not thousands, of data applications running over their networks, adequate bandwidth must be provisioned for this class as a whole to handle the sheer volume of applications that default to it. It is recommended that at least 25 percent of a WAN link’s bandwidth be reserved for the default Best-Effort class. Version 3.3
Chapter 3 WAN Aggregator QoS Design Bandwidth Provisioning for Real-Time Traffic Serialization Version 3.3 WAN Edge QoS Design Considerations Not only does the Best-Effort class of traffic require special bandwidth-provisioning consideration, but the Real-Time class does as well. The amount of bandwidth assigned to the Real-Time class is variable; however, if too much traffic is assigned to Real-Time (strict-priority/low-latency) queuing, the overall effect is a dampening of QoS functionality for data applications. The goal of convergence cannot be overemphasized: to enable voice, video, and data to coexist transparently on a single network. When real-time applications (such as voice or interactive-video) dominate a WAN link, data applications fluctuate significantly in their response times, destroying the transparency of the “converged” network. Cisco Technical Marketing testing has shown a significant decrease in data application response times when Real-Time traffic exceeds one-third of a link’s bandwidth capacity. Cisco IOS Software allows the abstraction (and, thus, configuration) of multiple LLQs. Extensive testing and production-network customer deployments have shown that limiting the sum of all LLQs to 33 percent is a conservative and safe design ratio for merging real-time applications with data applications. Furthermore, it should be kept in mind that if VoIP traffic is set to dominate a link via low-latency queuing (which is essentially strict-priority FIFO queuing), VoIP actually could negatively impact other VoIP traffic because of extensive FIFO queuing. This easily could result in excessive serialization delays (Š 10 ms per hop) on even medium-speed links (T1/E1 links) where serialization delays ordinarily would not even be a consideration. (Serialization delays are discussed in more detail in the next section.) Such excessive serialization delays from VoIP LLQ overprovisioning would increase VoIP jitter and, thus, decrease overall call quality. Note The 33-percent limit for the sum of all LLQs is simply a best-practice design recommendation; it is not a mandate. In some cases, specific business objectives cannot be met while holding to this recommendation. In such cases, enterprises must provision according to their detailed requirements and constraints. However, it is important to recognize the trade-offs involved with overprovisioning LLQ traffic in respect to the negative performance impact on data application response times. Serialization delay refers to the finite amount of time it takes to clock a frame onto the physical media. Within the campus, this time is so infinitesimal that it is completely immaterial. Over the WAN, however, lower link speeds can cause sufficient serialization delay to adversely affect real-time streams, such as Voice or Interactive-Video. Serialization delays are variable because they depend not only on the line rate of the link speed, but also on the size of the packet being serialized. Variable (network) delay also is known as jitter. Because the end-to-end one-way jitter target has been set as 30 ms, the typical per-hop serialization delay target is 10 ms (which allows for up to three intermediate hops per direction of VoIP traffic flow). This 10 ms per-hop target leads to the recommendation that a link fragmentation and interleaving (LFI) tool (either MLP LFI or FRF.12) be enabled on links with speeds at or below 768 kbps (this is because the serialization delay of a maximum-size Ethernet packet—1500 bytes—takes more than 10 ms to serialize at 768 kbps and below). Naturally, LFI tools need to be enabled on both ends of the link. When deploying LFI tools, it is recommended that the LFI tools be enabled during a scheduled downtime. Assuming that the network administrator is within the enterprise’s campus, it is recommended that LFI be enabled on the branch router first (which is on the far end of the WAN link) because this generally takes the WAN link down. Then the administrator can enable LFI on the WAN aggregator (the near end of the WAN link), and the link will come back up. Otherwise, if the Enterprise QoS Solution Reference Network Design Guide 3-3
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