Interface configuration of Media Converters vs. Switches

The behavior of Media Converters is quite different from the behavior of Switches (also known as “Bridging Media Converters”). These differences in behavior have important implications when one considers MIB variables dealing with physical layer features such as Speed, Auto-negotiation, Duplex, and Pause.

A Media Converter is a simple device. It is capable only of attaching two dissimilar physical interfaces to each other. The two interfaces connected by the Media Converter must have characteristics that are otherwise compatible – the Media Converter is not designed to resolve differences aside from the physical media. A Switch (a.k.a. “Bridging Media Converter”) is by comparison an intelligent device, typically capable of resolving a variety of physical layer incompatibilities (e.g. Speed and Duplex differences) between the interfaces being connected.

When a port on a Switch is configured, its features are manipulated in a fairly intuitive way: For example, when one enables “Full Duplex” on a Switch port, the Switch port begins supporting and advertising Full Duplex operation between the Switch port and the device to which that port is directly attached.

This differs significantly from the meaning of interface configuration on a standard Media Converter, which is not intelligent enough to actually ‘care’ about many physical layer features (such as duplex). Instead of actually enabling features on the interface, the configuration MIB variables merely cause the Media Converter to regenerate related advertisements* on the selected interface.

The desired effect is to render the pair of Media Converters ‘invisible’ to the end stations. i.e. the media converters should behave in a way that makes each end station ‘think’ that it is talking directly to the other end station. This is accomplished by configuring each Media Converter interface that is directly attached to an end station interface to exhibit the same behavior that the opposite end station interface exhibits. This nearly always involves setting both ‘outside’ Media Converter interfaces exactly the same way, since the two end station interfaces must be compatible in order for non-bridging Media Converters to work properly.

This re-generation of advertisements is necessary because not all media are capable of carrying all advertisements that are designed for other types of media. This is particularly true when the differences between the media are great, as they are when one is copper and the other is fiber.

* Please note that in the interest of brevity, this document uses the term “advertisements” in a fairly sloppy fashion. What is really meant is “advertisements or other electrical behaviors (e.g. link pulses) that communicate interface characteristics to the directly attached device.”