Medianet Reference Guide - Cisco

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.
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Medianet Reference Guide
OL-22201-01