Ring structures
are prohibited to prevent endless circulation of errored, rogue, or misdirected packets.
There are Ethernet devices that can be connected in a ring configuration, however they
do not conform to the IEEE 802.3 protocol in this respect.
Ring topologies have practical benefits and are frequently deployed in carrier networks.
They, (1) uses less fiber than mesh or hub and spoke configurations and (2) in
most cases are simpler and quicker to restore in the event of failure.
Slow and Non-deterministic Restoration
A key consideration in carrier networks is protection and restoration. Ethernet has
protection mechanisms, Spanning Tree and Rapid Spanning Tree, but these hierarchical
and mesh restoration schemes do not converge fast enough for networks with strict
Service Level Agreements.
356 Chapter 12
Protection and topology discovery protocols are tightly related to each other.
Protection information is used to update the topology data base, and a common frame
carries information relevant to both functions. Topology discovery provides a reliable
and accurate means for all stations on a ring to discover the topology of the stations
on the ring, and any changes to that topology.
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