BRIDGES

 
BRIIDGES
Bridges are used to extend or interconnect LAN segments,
whether the segments consist of wired or wireless links. At
one level, they are used to create an extended network that
greatly expands the number of devices and services available
to each user. At another level, bridges can be used for
segmenting LANs into smaller subnets to improve performance,
control access, and facilitate fault isolation and testing
without impacting the overall user population.
The bridge does this by monitoring all traffic on the subnets
that it links. It reads both the source and destination
addresses of all the packets sent through it. If the bridge
encounters a source address that is not already contained in
its address table, it assumes that a new device has been
added to the local network. The bridge then adds the new
address to its table.
In examining all packets for their source and destination
addresses, bridges build a table containing all local addresses.
The table is updated as new packets are encountered and as
addresses that have not been used for a specified period of time keep up with changes on the network without requiring that
their tables be updated manually.
The bridge isolates traffic by examining the destination
address of each packet. If the destination address matches
any of the source addresses in its table, the packet is not
allowed to pass over the bridge because the traffic is local.
If the destination address does not match any of the source
addresses in the table, the packet is discarded onto an
adjacent network. This filtering process is repeated at
each bridge on the internetwork until the packet eventually
reaches its destination. Not only does this process
prevent unnecessary traffic from leaking onto the internetwork,
it acts as a simple security mechanism that can
screen unauthorized packets from accessing various corporate
resources.
Bridges also can be used to interconnect LANs that use
different media, such as twisted-pair, coaxial, and fiberoptic
cabling and various types of wireless links. In office
environments that use wireless communications technologies
such as spread spectrum and infrared, bridges can
function as an access point to wired LANs (Figure B-3). On
the widea area network (WAN), bridges even switch traffic
to a secondary port if the primary port fails. For example,
a full-time wireless bridging system can establish a
modem connection on the public network if the primary
wire or wireless link is lost because of environmental
interference.
In reference to the Open Systems Interconnection (OSI)
model, a bridge operates at Layer 2; specifically, it operates
at the Media Access Control (MAC) sublayer of the
Data Link Layer. It routes by means of the Logical Link
Control (LLC), the upper sublayer of the Data Link Layer
(Figure B-4).
Because the bridge connects LANs at a relatively low
level, throughput often exceeds 30,000 packets per second
BRIDGES 29
(pps). Multiprotocol routers and gateways, which provide
LAN interconnection over the WAN, operate at higher levels
of the OSI model and provide more functionality. In performing
more protocol conversions and delivering more functionality,
routers and gateways are generally more
processing-intensive and, consequently, slower than bridges.