Medianet Reference Guide - Cisco

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Bandwidth Over Subscription Chapter 2 Medianet Bandwidth and Scalability on the wire. The next device upstream sees an incoming microburst twice as large as normal. If the RxRing saturates, it is possible to begin dropping packets at very modest 1 second average loads. As more video is added, the probability that multiple frames will converge increases. This can also load Tx Queues, especially if the multiple high speed source interfaces are bottlenecking into a single low-speed WAN link. Another concern is when service provider policers cannot accept large, or back-to-back bursting. Video traffic that may naturally synchronize frame transmission is of particular concern and is likely to experience drops well below 90 percent circuit utilization. Multipoint TelePresence is a good example of this type of traffic. The Cisco TelePresence Multipoint Switch replicates the video stream to each participant by swapping IP headers. Multicast interfaces with a large fanout are another example. These types of interfaces are often virtual WAN links such as Dynamic Multipoint Virtual Private Network (DMVPN), or virtual interfaces such as Frame Relay. In both cases, multipoint flows fanout at the bandwidth bottleneck. The same large packet is replicated many times and packed on the wire close to the previous packet. Buffer and queue depths of the Tx interface can be overrun. Knowing the queue buffer depth and maximum expected serialization delay is a good method to determine how much video an interface can handle before drops. When multiple video streams are on a single path, consider the probability that one frame will overlap or closely align with another frame. Some switches allow the user some granularity when allocated shared buffer space. In this case, it is wise to ensure buffers that can be expected to process long groups of real-time packets and have an adequate pool of memory. This can mean reallocating memory away from queues where packets are very periodic and groups of packets are generally small. For now, some general guidelines are presented as the result of lab verification of multipoint TelePresence. Figure 2-8 shows both the defaults and tuned buffer allocation on a Cisco Catalyst 3750G Switch. Additional queue memory has been allocated to queues where tightly spaced packets are expected. By setting the buffer allocation to reflect the anticipated packet distribution, the interface can reach a higher utilization as a percent of line speed. 2-14 Medianet Reference Guide OL-22201-01
Chapter 2 Medianet Bandwidth and Scalability Capacity Planning Figure 2-8 Default and Tuned Buffer Allocation Default Values Tuned Values 0x200000 0x180000 0x100000 0x080000 Output particle memory pool Q4 25% Q3 25% Q2 25% Q1 25% Input particle memory pool Q2 50% Q1 50% 0x0C0000 0x060000 0x200000 0x1E6666 0x133333 0x099999 Output particle memory pool Q4 - 5% Q3 35% Q2 30% Q1 30% Input particle memory pool Q2 - 50% Q1 70% 0x0C0000 0x086666 0x000000 0x000000 0x000000 0x000000 4:1 Interface ASIC 4:1 Interface ASIC Ports Ports 228643 It may take some fine tuning to discover the values most appropriate to the load placed on the queues. Settings depend on the exact mix of applications using the interface. Capacity Planning Capacity planning involves determining the following: • How much video is currently running over the network • How much future video is expected on the network • The bandwidth requirements for each type of video • The buffer requirements for each type of video The first item above is discussed in Chapter 6, “Medianet Management and Visibility Design Considerations.” Tools available in the network such as NetFlow can help understand the current video loads. The future video requirements can be more subjective. The recent trend is for more video and for that video to be HD. Even if the number of video streams stays the same, but is updated from SD to HD, the video load on the network will grow substantially. OL-22201-01 Medianet Reference Guide 2-15
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Medianet Reference Guide - Cisco

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