Understanding the network.pdf - Back to Home

or performance limitations, network administrators had two choices:
extend the Layer 2 network segment with a bridge, or partition (or
add) an additional network segment with a segmentation router. Both
of these solutions created as many problems as they solved.
In the solution column, routers and bridges provide similar functions
by two different means: Bridges operate at Layer 2, and routers
operate at Layer 3. Functionally, however, they both can provide
translation service to different Layer 2 transport media. They both can
be used to reduce CSMA/CD collision domains, which are extended by
the use of Ethernet repeaters. And, of course, they both enable the
network to expand to its theoretical operational and/or performance
limitation again.
The major downside to growing the network occurs irrespective of
whether a router or bridge is used: increased network latency.
Latency is the amount of time it takes for the first bit of the packet to
reach the destination interface and the last bit of the packet to leave
the source interface. Because both bridges and routers store or
process the packets for a period of time, this increases the overall
packet delivery time between hosts. Latency is unavoidable in large
networks, because they are dependent on bridges and routers to relay
data between end-stations. However, the amount of latency between
a router and a bridge can be significant, and it is a direct result of the
communication layer they operate on.
Routing requires the Layer 3 datagram to be extracted from the Layer
2 packet, stimulate a route lookup, and then place the datagram into
another Layer 2 packet to be forwarded on when the wire is available.
A bridge only copies the packet, checks its SAT, filters it (discards it),
or forwards it on when the wire is available. Both, of course, are
subject to wire availability, so they have storage buffers that enable
them to process packets/datagrams while waiting to transmit. The
router, by comparison, requires more processing time than the bridge,
and therefore represents a source of higher network latency. Routers
also add complexity and additional configuration to the network.
Because routers function at Layer 3, their implementation requires
that end-stations on different parts of the network utilize different
network addressing. Router redundancy is also harder to accomplish
than Layer 2 redundancy from an end-station perspective.
In terms of LAN implementation, routers and bridges are affected by
similar factors, mainly proper placement. When installing a router or a
bridge, the most important performance factor is placement. Ideally,
common user end-stations and network resources (servers, printers,