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The dynamic address assignment allows one to create IP networks in which the number of nodes
exceeds the number of the IP addresses administrator has.
4.2.3 Bridging of local networks
Bridges are the simplest devices for logical network structuring. They divide the transmission
network medium into segments (logical segments), forwarding data from one segment to another,
if such a transmission is necessary, i.e. if the destination address belongs to another subnet.
Bridges are data communication devices that operate at the data link layer of the OSI reference
model. They use addresses of computers and other devices. Bridges control data flow, handles
transmission errors, provides physical (as opposed to logical) addressing and manage access to
the physical medium. Bridges provide these functions by using various link-layer protocols that
dictate specific flow control, error handling, and addressing and medium-access algorithms.
The primary advantage of bridging is the upper-layer protocol transparency. Because bridges
operate at the data link layer, they are not required to examine upper layer information. It means
that that they can rapidly forward traffic representing any network layer protocol.
By dividing large networks into self-contained units, bridges provide a range of additional
advantages. First, because only a certain percent of traffic is forwarded, bridges diminish traffic
passing through devices of all connected segments. Second, bridges act as a firewall for some
potentially damaging network errors. Third, bridges allow communication between a larger
number of devices than any single LAN connected to the bridge would support. Fourth, bridges
extend the effective LAN length, permitting the attachment of distant stations.
Types of bridges
Bridges can be either local or remote. Local bridges provide a direct connection of subnet
segments in the same area. Remote bridges connect subnet segments in different areas, usually
over telecommunication lines. The MDS92xxx-10BT, device belongs to remote bridges.
Remote bridging represents several unique internetworking challenges. One of them is the
difference between LAN and WAN speeds. Vastly different LAN and WAN speeds sometimes
prevent users from running delay-sensitive network applications over the WAN.
Remote bridges cannot increase WAN speeds, but they can compensate for the speed
discrepancies by using buffering capacities. If a LAN device capable of a 10-Mbit/s transmission
rate intends to communicate with another remote LAN device, the local bridge must regulate the
10-Mbit/s information flows in order not to overwhelm the 2-Mbit/s serial link. It is done by storing
the incoming data in buffers and transmitting it over a serial link. This can be achieved only for
short bursts of data that do not overwhelm the bridge’s buffering capacity.
The MDS92xxx-10BT device implements “transparent bridge” and “spanning tree” algorithms.
The “transparent bridge” is called so because its presence and operation is transparent to all
network hosts.
A bridge builds its own address table while passively monitoring the traffic. At this stage it
extracts the information about source addresses of data frames. The source address shows that
it belongs to a certain node of this or that network segment. Fig. 2 shows the creation of an
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