2.5: Ethernet Switches

An Introduction to Computer Networks · LibreTexts

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    From LibreTexts, under CC BY-NC-ND 4.0, as the source states it; its equations are written in TeX, which the PDF shows typeset. This page at LibreTexts.

    Switches join separate physical Ethernets (or sometimes Ethernets and other kinds of networks). A switch has two or more Ethernet interfaces; when a packet is received on one interface it is retransmitted on one or more other interfaces. Only valid packets are forwarded; collisions are not propagated. The term collision domain is sometimes used to describe the region of an Ethernet in between switches; a given collision propagates only within its collision domain.

    Switches have revolutionized Ethernet layout: all the collision-detection rules, including the rules for maximum network diameter, apply only to collision domains, and not to the larger “virtual Ethernets” created by stringing collision domains together with switches. As we shall see below, a switched Ethernet also offers much more resistance to eavesdropping than a non-switched (eg hub-based) Ethernet.

    Switch Costs

    In the 1980’s the author once installed a two-port 10-Mbps Ethernet switch (then called a “bridge”) that cost $3000; cf the [MB76] quote at 2 Ethernet. Today a wide variety of multiport 100-Mbps Ethernet switches are available for around $10, and almost all installed Ethernets are fully switched.

    Like simpler unswitched Ethernets, the topology for a switched Ethernet is in principle required to be loop-free. In practice, however, most switches support the spanning-tree loop-detection protocol and algorithm, 2.5 Spanning Tree Algorithm and Redundancy, which automatically “prunes” the network topology to make it loop-free while allowing the pruned links to be placed back in service if a primary link fails.

    While a switch does not propagate collisions, it must maintain a queue for each outbound interface in case it needs to forward a packet at a moment when the interface is busy; on (rare) occasion packets are lost when this queue overflows.

    This page titled 2.5: Ethernet Switches was last modified on Tue, 01 Dec 2020 03:51:14 GMT and is shared under a CC BY-NC-ND 4.0 license and was authored, remixed, and/or curated by Peter Lars Dordal.