IPTV and the Spanning Tree Process

By Phil Hippensteel

IPTVDear Professor Phil,

We’re using IPTV at our school and have discovered a situation that we don’t understand. The video is delivered over a group of Ethernet switches. When we investigate the individual links, we find some links carry large amounts of traffic, while others transport no traffic at all. Can you offer a possible explanation?
Barry, Alexandria, VA

Barry,

A very likely explanation is that the switches have a loop circuit within them and the spanning tree process has automatically disabled a link. That’s not bad. In fact, it is a very necessary requirement.

IPTVThe spanning tree algorithm is used with Ethernet switches to routinely remove loop circuits. If these loops weren’t removed, a single IP packet carrying video could loop endlessly while consuming valuable bandwidth. To understand essentially how spanning tree works, look at the diagram of a hypothetical network of switches. The simple hexagon with a diagonal has three loops in it: A-D-E-F-A, A-D-C-B-A, and A-B-C-D-E-F-A.

The spanning tree algorithm picks a switch, say B, to be the root of a tree that will be formed. A series of spanning tree messages is sent over the links starting from the root switch, B. These messages allow the switches to discover that a loop exists. For example, in our drawing, switch E received messages from both F and D. Therefore, E disables the link to one of them, say the one to D. In the same manner, C chooses to disable its link to D. After these two operations, the loops are eliminated and the entire structure forms a tree. Now every switch is connected to every other switch through exactly one path. Occasionally, technicians disable the spanning tree operation to reduce traffic, but that is almost always a bad idea.

Phil Hippensteel, Ph.D., has spent more than forty years in higher education and now teaches at Penn State Harrisburg.

About Jim Jachetta

Jim Jachetta, EVP & CTO of VidOvation Corporation, has over 25 years of experience in designing, integrating and delivering video audio and data communications systems over wireless, IP networks and fiber optics. Jim drives VidOvation to create solutions using world class technology to make the “impossible” and “never been done before” a cost effective, everyday solution aligned with your business goals – modern technology, creatively implemented to meet your business needs, easy to support, and delivered at a price point that leads the industry.

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