Enterprise QoS Solution Reference Network Design Guide

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WAN Edge Classification and Provisioning Models 3-10 Enterprise QoS Solution Reference Network Design Guide Chapter 3 WAN Aggregator QoS Design class Critical Data bandwidth percent 36 ! Critical Data class gets 36% BW guarantee random-detect dscp-based ! Enables DSCP-WRED for Critical-Data class class Scavenger bandwidth percent 1 ! Scavenger class is throttled class class-default bandwidth percent 25 ! Default class gets a 25% BW guarantee random-detect ! Enables WRED for class-default ! Verification command: show policy High Link Speed QoS Class Models High-speed links (such as multiple T1/E1 or above speeds) allow for the provisioning of Voice, Interactive-Video, and multiple classes of data, according to the design rules presented in this chapter (for example, 25 percent for Best Effort class and < 33 percent for all LLQs). Enabling QoS only optimizes the efficiency of bandwidth utilization; it does not create bandwidth. Therefore, it is important to have adequate bandwidth for all the applications being provisioned. Furthermore, as WAN bandwidth is becoming less expensive, higher-speed links are becoming more popular. Even if adequate bandwidth exists for up to 11 classes of traffic, as outlined by the QoS Baseline Model, not all enterprises are comfortable with deploying such complex QoS policies at this time. Therefore, it is recommended to start simple, but with room to grow into more complex models. Figure 13-4 illustrates a simple migration strategy showing which classes are good candidates for subdivision into more granular classes as future needs arise. Figure 3-4 Number of QoS Classes Migration Strategy Example Five-Class Model Real-Time Call-Signaling Critical Data Best-Effort Scavenger Eight-Class Model Voice Video Call-Signaling Network-Control Critical Data Bulk Data Best-Effort Scavenger QoS Baseline Model If the enterprises’ QoS requirements exceed that which the Five-Class Model can provision for (such as requiring service guarantees for Interactive-Video and requiring Bulk Data to be controlled during busy periods), they might consider migrating to the Eight-Class Model. Voice Interactive-Video Streaming-Video Call-Signaling IP Routing Network-Management Mission-Critical Data Transactional Data Bulk Data Best-Effort Scavenger Version 3.3
Chapter 3 WAN Aggregator QoS Design Eight-Class Model Version 3.3 WAN Edge Classification and Provisioning Models The Eight-Class Model introduces a dual-LLQ design: one for Voice and another for Interactive-Video. As pointed out in Chapter 5, the LLQ has an implicit policer that allows for time-division multiplexing of the single priority queue. This implicit policer abstracts the fact that there is essentially a single LLQ within the algorithm and, thus, allows for the “provisioning” of multiple LLQs. Interactive-video (or IP videoconferencing, known also as IP/VC) is recommended to be marked AF41 (which can be marked down to AF42 in the case of dual-rate policing at the campus access edge). It is recommended to overprovision the LLQ by 20 percent of the IP/VC rate. This takes into account IP/UDP/RTP headers as well as Layer 2 overhead. Additionally, Cisco IOS Software automatically includes a 200-ms burst parameter (defined in bytes) as part of the priority command. On dual-T1 links, this has proven sufficient for protecting a single 384-kbps IP/VC stream; on higher-speed links (such as triple T1s), the default burst parameter has shown to be insufficient for protecting multiple IP/VC streams. However, multiple-stream IP/VC quality tested well with the burst set to 30,000 bytes (for example, priority 920 30000). Our testing did not arrive at a clean formula for predicting the required size of the burst parameters as IP/VC streams continually were added; however, given the variable packet sizes and rates of these Interactive-Video streams, this is not surprising. The main point is that the default LLQ burst parameter might require tuning as multiple IP/VC streams are added (which likely will be a trial-and-error process). Optionally, DSCP-based WRED can be enabled on the Interactive-Video class, but testing has shown negligible performance difference in doing so (because, as already has been noted, WRED is more effective on TCP-based flows than UDP-based flows, such as Interactive-Video). In these designs, WRED is not enabled on classes such as Call-Signaling, IP Routing, or Network-Management because WRED would take effect only if such classes were filling their queues nearly to their limits. Such conditions would indicate a provisioning problem that would better be addressed by increasing the minimum bandwidth allocation for the class than by enabling WRED. Additionally, the Eight-Class Model subdivides the preferential data class to separate control plane traffic (IP routing and Network-Management applications) from business-critical data traffic. Interior Gateway Protocol (such as RIP, EIGRP, OSPF, and IS-IS) packets are protected through the PAK_priority mechanism within the router. However, EGP protocols, such as BGP, do not get PAK_priority treatment and might need explicit bandwidth guarantees to ensure that peering sessions do not reset during periods of congestion. Additionally, administrators might want to protect network-management access to devices during periods of congestion. The other class added to this model is for bulk traffic (Bulk Data class), which is also spun away from the Critical Data class. Because TCP continually increases its window sizes, which is especially noticeable in long sessions (such as large file transfers), constraining Bulk Data to its own class alleviates other data classes from being dominated by such large file transfers. Bulk Data is identified by DSCP AF11 (or AF12, in the case of dual-rate policing at the campus access edges). DSCP-based WRED can be enabled on the Bulk Data class (and also on the Critical Data class). Figure 3-5 shows sample bandwidth allocations of an Eight-Class Model (for a dual-T1 link example). Figure 3-5 also shows how this model can be derived from the Five-Class Model in a manner that maintains respective bandwidth allocations as consistently as possible, which increases the overall end-user transparency of such a migration. Enterprise QoS Solution Reference Network Design Guide 3-11
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