On Count-to-Infinity Induced Forwarding Loops Ethernet Networks

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Abstract
Ethernet's high performance, low cost and ubiquity have made it the dominant networking technology for many application domains. Unfortunately, its distributed forwarding topology computation protocol - the Rapid Spanning Tree Proto- col (RSTP) - can suffer from a classic "count-to-infinity" pr oblem that may lead to a forwarding loop under certain network failures. The consequences are serious. During the period of "count-to-infinity", which can last tens of seconds even in a small network, the network can become highly congested by packets that persist in cycles in the network, even packet forwarding can fail as the forwarding tables are polluted. In this paper, we explain the origin of this problem in detail and study its behavior. We find that simply tuning RSTP's parameter settings cannot adequately address the fundamental problem with "count-to- infinity". We propose a simple and effective solution called RSTP with Epochs. This approach uses epochs of sequence numbers in protocol messages to eliminate stale protocol information in the network and allows the forwarding topology to recover in merely one round-trip time across the network.

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