If a failure were to occur somewhere on the ring then the spanning tree protocol will need to converge the topology which can take longer then desired.
Therefore, the topology under normal circumstances would be a long line of Ethernet switches. If one switch is the root of the spanning tree then another switch somewhere on the opposite site of the ring will have a port leading to the root in blocking mode. This 7-hop Spanning Tree limit is not to be confused with the 5-4-3 rule of thumb for 10Mbps Ethernet (5 total segments, 4 repeater hops, and 3 user-populated segments).īelow is a diagram of a ring Ethernet network. The default STP timers are set with the expectation that the diameter is no more than 7 hops. The second issue that you encounter is the 7-hop maximum dimension of spanning tree. Service providers would prefer optical convergence times on the order of 50ms which is what they expect from their DWDM rings. When a service provider tries delivering converged triple-play services the 30 seconds to 50 seconds of convergence is unacceptable. The traditional spanning tree timers (15 seconds for listening, 15 seconds for learning, and 20 seconds max-age timeout) don’t allow for fast convergence times that measure up to today’s standards for converged network traffic. The first issue you run into with spanning tree is the convergence times.
We are all familiar with the limitations of traditional IEEE 802.1D spanning tree. If you have ever thought about building a ring topology using Ethernet links you quickly come to realize the limitations of traditional spanning tree. These new extensions to spanning tree allow for fast convergence of Ethernet ring topologies.
With the pervasiveness of Ethernet technology you may find yourself contemplating using Ethernet with a ring architecture. Ethernet is traditionally thought of a as a bus or star topology. If you are considering a network design that has a ring topology and you desire to use Ethernet then you should first be aware of the issues of using Ethernet with a ring architecture.
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