LAN INTERWORKING VIA SPANNING DEVICES
traffic introduced into this network will circulate indefinitely around the loop created by
the two repeaters. On larger networks the effects can be devastating, although perhaps
less apparent (Ref. 9).
Whereas repeaters have no intelligence, bridges do. Bridges can connect two LANs, at
the data-link or MAC protocol level. There are several varieties of bridges, depending on
the intelligence incorporated.
There is the transparent bridge that builds a list of nodes the bridge sees transmitting
on either side. It isolates traffic and will not forward traffic that it knows is destined to
another station on the same side of the bridge as the sending station. The bridge is able to
isolate traffic according to the MAC source and destination address(es) of each individual
data frame. MAC-level broadcasts, however, are propagated through the network by the
bridges. A bridge can be used for segmenting and extending LAN coverage. Thus it
lowers traffic volume for each segment. A transparent bridge does not modify any part
of a message that it forwards.
The second bridge is the translation bridge. It is used to connect two dissimilar LANS,
such as a token ring to CSMA/CD. In order to do this it must modify the MAC-level header
and FCS of each frame it forwards in order to make it compatible with the receiving LAN
segment. The MAC addresses and the rest of the data frame are unchanged. Translation
bridges are far less common than transparent bridges.
The third type of bridge, as shown in Figure 11.15, is the encapsulation bridge. It is also
used to connect LANs of dissimilar protocols. But rather than translate the MAC header
and FCS fields, it simply appends a second MAC layer protocol around the original frame
for transport over the intermediate LAN with a different protocol. There is the destination
bridge which strips off this additional layer and extracts the original frame for delivery
to the destination network segment.
The fourth type of bridge is a source routing bridge. It is commonly used in token
ring networks. With source routing bridges, each frame carries within it a route identifier
The concept of bridging. Top: encapsulation bridge; bottom: source routing bridge.
(Courtesy of Hewlett-Packard Co., Ref. 9.)