Ethernet Switching Protocols

The protocol originally employed to make forwarding decisions in Ethernet networks was the Spanning Tree Protocol (STP). Early versions of STP were not sensitive to service interruptions. For example, a physical topology change often resulted in temporary cessation of frame forwarding that could last up to one minute. That historical behavior contributes to the public's lingering misunderstanding of modern Ethernet networks. Many enhancements have been made to STP over the years. In particular, physical topology changes are now detected and resolved much more quickly. In properly designed Ethernet networks, frame-forwarding disruptions of less than 1 second can be achieved during physical topology changes. Furthermore, STP was officially replaced by the Rapid Spanning Tree Protocol (RSTP) in 2004. RSTP improves certain operating characteristics of STP while maintaining backward compatibility with STP. So, switches running STP can be connected to switches running RSTP, thus facilitating phased migration. However, the full benefits of RSTP are not available in mixed networks. The configuration restrictions are identical for both protocols.

RSTP is a variation of the distance vector model. Each switch learns the location of MAC addresses by inspecting the Source Address field in the header of Ethernet frames received on each switch port. Thus, RSTP operation is completely transparent to end nodes. The learned addresses are entered into a forwarding table that associates each address with an egress port (the port on which the address was learned). No information is stored regarding the distance to each address. So, RSTP is a vector based protocol. No information is exchanged between switches regarding the reachability of MAC addresses. However, switches do exchange information about the physical topology so that loop suppression may occur. In multi-VLAN environments, the Multiple Spanning Tree Protocol (MSTP) may be used. MSTP is a variation of RSTP that enables an independent spanning tree to be established within each VLAN on a common physical network.

Because none of the Ethernet header formats include a TTL field (or equivalent), Ethernet frames can be forwarded indefinitely in topologies that have one or more loops. For example, when a broadcast frame (such as an ARP request) is received by a switch, the switch forwards the frame via all active ports except for the ingress port. If a loop exists in the physical topology, the forwarded broadcast frame eventually returns to the same switch and is forwarded again. Meanwhile, new broadcast frames are generated by the attached nodes. Those frames are forwarded in the same manner as the first broadcast frame. This cycle continues until all available bandwidth on all active ports is fully consumed by re-circulating broadcast frames. This phenomenon is called a broadcast storm. RSTP suppresses all loops in the physical topology to prevent broadcast storms and other congestion-related failures. The resulting logical topology is a tree that spans to facilitate connectivity between all attached nodes. Connectivity is always symmetric, which means that frames exchanged between a given pair of end nodes always traverse the same path in both directions. For more information about Ethernet switching protocols, readers are encouraged to consult the IEEE 802.1Q-2003 and 802.1D-2004 specifications.