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Currently, two different bridging technologies are in widespread use.
''Transparent bridging'' predominates in [[Ethernet]] networks;
while ''source
==Transparent Bridging==
''Transparent bridging'' refers to a form of bridging "transparent" to the
end systems using it, in the sense that the end systems operate in
exactly the same way as in a non-bridged network.
It is used primarily in Ethernet networks, where it has
been standardized as [[IEEE 802.1d]].
The bridging
functions are confined to [[network bridges]] which interconnect
the [[network segments]]. First, the network bridges use
a [[spanning-tree]] algorithm to built a loop-free network
topology by selectively disabling network segments.
This allows broadcasting
to occur simply by copying packets;
the tree structure insures that loops will
not occur and that broadcast packets will
therefore not be copied indefinately.
Next, the bridges monitor all packets traveling
on the network, noting their source addresses,
matching these addresses to the interfaces
the packets were received on, and recording this
information in a table. Finally, the bridges
forward packets according to a simple algorithm
- if the destination address does not appear
in the table, broadcast the packet throughout
the network; otherwise, forward it only on the
interface recording in the table.
Note that both source and destination addresses
are used in this algorithm. Source addresses
are used to record entries in the table, while
destination addresses are used to lookup entries.
As a simple example, consider a packet exchange between
two hosts (A and B) in a bridged network.
A begins by transmitting a packet to B.
Since B (the destination) does not appear in
any of the bridges's tables, the packet
is broadcast throughout the network.
Each network bridge sees a copy of the packet,
and records its source address (A) along
with the interface it was received on.
Each bridge now has an entry for A in its
table. B, having received a packet from A,
now transmits a packet in response. Since
the bridges all have an entry for A (the
packet's destination), no broadcast is
required, and the packet is forwarded
along the tree from B to A. All the bridges
along this path record the packet's source
address - B. Two-way communication is
now possible between A and B without any
further broadcasting. Note, however, that
only the bridges along the direct path
between A and B possess table entries for B.
If a third host (C), not along the direct
path, now transmits a packet for B, it
will be broadcast throughout
the network.
==Source Route Bridging==
''Source route bridging'' is used primarily on
[[Token-Ring]] networks, and
is standardized in Section 9 of the [[IEEE 802.2]]
standard. The [[spanning-tree]]
algorithm is not used, the operation of the
[[network bridges]] is simplier, and much of
the bridging functions are performed by the
end systems, particularly the sources, giving rise to its name.
A field in the Token-Ring header, the
Routing Information Field (RIF) is used to
support source-route bridging. Upon sending
a packet, a host attaches a RIF to the packet
indicating the series of bridges and network segments
to be used for deliverying the packet to its
destination. The bridges mearly follow the list
given in the RIF - if a given bridge is next
in the list, it forwards the packet, otherwise it
ignores it.
When a host wishes to send a packet to a destination
for the first time, it needs to determine an appropriate
RIF. A special type of broadcast packet is used,
which instructs the network bridges to append their
bridge number and network segment number to each
packet as it is forwarded. Loops are avoided by
requiring each bridge to ignore packets which
already contain its bridge number in the RIF field.
At the destination, these broadcast packets are
modified to be standard unicast packets and returned
to the source along the reverse path listed in
the RIF. Thus, for each route discovery packet
broadcast, the source receives back a set of
packets, once for each possible path through
the network to the destination. It is then
up to the source to choice one of these paths
(probably the shortest one) for further
communications with the destination.
== Source Routing Transparent (SRT) Bridging==
''Source routing transparent'' (SRT) bridging
is a hybrid of source routing and transparent
bridging, standardized in Section 9
of the [[IEEE 802.2]] standard.
It allows source routing and transparent
bridging to coexist on the same
bridged network by using source routing
with hosts than support it and transparent
bridging otherwise.
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