Uses of Bridges
1) Many commercial department and institution of higher education have
their own LANs, which are used to interconnect personal computers,
workstations and servers. Different commercial departments have
different goals so they choose different LANs without considering to
other departments work. Bridges are used if there is need for
2) The organization may be geographically spread by distances over
several buildings. It may be costless to have separate LANs in each
building and interconnect them with Bridges and infrared links that to
run a single coaxial cable over the entire site.
3) It may be necessary to divide a single LAN into separate LANs to hold
the load. Files are normally placed on file server’s machines and are
downloaded to user machines upon request. The vast scale of this system
prevents putting all the workstations on a single LAN so total bandwidth
required is far too high. As an alternative multiple LANs connected by
bridges are used. Each LAN includes a cluster of workstations with its
own file server, so that most traffic is limited to a single LAN and
does not add load to the backbone.
4) A single LAN would be sufficient in terms of the load but the
physical distance between the most distant machines is too large. Even
if placing the cable is easy to do, the network would not operate due to
the large round-trip delay. The only solution is to divide the LAN and
install bridges between segments. By using bridges the total physical
distance required can be increased.
5) For reliability purpose, Bridges are used. A defective node on a
single LAN that keeps outputting a continuous stream of garbage will
cripple the LAN. Bridges can be placed at critical places like fire
doors in a building to protect a single node, which has gone berserk
from bringing down the entire system.
6) Bridges also provide organization’s security. Most LAN interfaces
have immoral mode, in which all frames are sent to the computer not just
addressed to it. By placing bridges at various points and being careful
not to forward it to a sensitive traffic, it is possible to separate
parts of the network so that its traffic cannot escape and fall into the
Different types of Bridges
1) Transparent Bridges
The first 802 bridge is known as transparent bridge or spanning Tree
Bridge. A transparent bridge works in promiscuous mode, accepting every
frame delivered all the LANs to which it is connected. When a frame
receives, a bridge must determine whether to remove or forward it and on
which LAN to put the frame. This is done by checking the destination
address in a big table inside the bridge. The table lists each
destination address and tells which output line it belongs on.
The entire hash table is empty when the bridges are first plugged in.
Bridge does not know where destinations of frame are so they make use of
flooding algorithm: every incoming frame for an unknown destination is
output on all the LANs to which the bridge is interconnected except the
one it arrived on. As time progresses the bridges learn where the
destinations are. Once a destination is known in advance, frames
destined for it are put on only the proper LAN and are not flooded.
Backward learning algorithm is used by transparent bridges. As bridges
work in immoral mode so they see every frame sent on any of their LANs.
By checking the source address, they can tell which machine is available
on which LAN. Machines and bridges are powered up and down and moved
around as the topology change takes place. Whenever a hash table entry
is made to handle dynamic topologies the arrival time of the frame is
noted in the entry. When a frame whose destination is already present in
the table arrives, its entry is updated with the current time. Thus the
time related with every entry tells the last time a frame from that
machine was seen.
2)source routing bridge
Source routing bridges considers that the sender of each frame knows
whether or not the destination is on its own LAN. While delivering a
frame to a different LAN, the source machine sets the high-order bit of
the source address to 1 to mark it. It is present in the frame header
the exact path that the frame will follow. This path is created as
follows. Each LAN consists of a unique 12-bit number, and each bridge
consists of 4-bit number that uniquely specifies it in the contexts of
its LANs. The two bridges far apart may both consists number 3 but two
bridges between the same two LANs must consists different bridge
numbers. A route is then order of sequence of bridge, LAN, bridge, LAN,
A source routing bridges are only considered those frames having
higher-order bit of the destination set to 1. For each such frame it
sees and checks the route looking for the number of the LAN on which the
frame arrived. If this LAN number is followed by its own bridge number,
the bridge forwards the frame onto the LAN whose number follows its
bridge number in the route.