Alcatel Omni Switch/Router, Руководство пользователя

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Page 18-1
18 Configuring
Frame Translations
Any-to-Any Switching
Because the Omni Switch/Router is a LAN switch that carries frames from multiple media
types on its backplane fabric, it offers the facility to switch frames from any media to any
other media. For example, an Ethernet frame onto a Token Ring. This feature is referred to as
Any to Any Switching.
Normally, the only way for data to get from one media type to another is via routing. Routing
removes the media specific headers of a received frame and prepends the new media specific
aspects of the destination port before the frame is retransmitted on the new media. In this
process the frame itself is not transmitted from one media to another, only the information
within it. This process involves heavy computation, requiring table lookups to guide the
header deletion/creation and additional router-to-router protocols to set up and maintain
these tables.
Routing is not restricted, nor even primarily intended, for moving data between unlike media
but instead seeks to break networks down into a number of smaller networks, each of which
is a broadcast domain. Historically, networks based on different technologies and media natu-
rally form distinct broadcast domains.
The advent of LAN switching has rewritten these rules. Today, the formation of broadcast
domains and the allocation of devices to them is driven by logical requirements such as
Virtual LANs and LAN switches. They seek to break free of topology and network constraints
imposed by mere media differences.
Within this new paradigm there is still a place for routing. The installed base of clients and
servers must communicate by established routing protocols but the broadcast domains
handled by a router need not now consist of a single media.
To support this paradigm a LAN switch must “transform” a frame on one media into a frame
on the other media in such a way that the frame is still acceptable to the routing protocols.
Unfortunately, the requirements for this “transformation” algorithm are specific to the various
protocols that currently exist. There is no single, simple algorithm that will allow the frame to
be switched between media transparently to the higher level protocols and frame formats.
This leads to a fairly complex set of configuration options and limitations on the applicability
of the any to any switching features.