Routers
A router is a device that directs (routes) data from one
path to another in a network. Routers base their switching information on one or
more information parameters contained within the packet of data they receive.
These parameters may include the destination address, availability of a
transmission path or communications channel, maximum allowable amount of
transmission delay a packet can accept, along with other key parameters. Routers
that connect data paths between different types of networks are sometimes called
gateways.
Routers provide some of the same functionality as network
switches. Their primary function is to provide a path for each routable packet
to its destination. When a router is initially installed into a network, it
begins its life by requesting a data network address. Using this data network
address, it sends (broadcasts) messages to nearby routers and begins to store
address connections of routers that are located around it. Routers regularly
exchange their connection information (lists of devices it is connected to) with
nearby routers to help them keep the latest packet routing information.
A router can make decisions on where to forward packets dependent
on a variety of factors including the maximum transmission distance or packet
priority. Distance vector routing and link state routing allow the router to
select paths that match the needs of the data that is being sent through it.
Some routers may use fixed (static) routing tables that are
manually programmed by the network administrator instead of dynamically created
routing tables. The use of static routing tables may seem to be inflexible,
however the use of static routing ensures other router’s that may have corrupt
routing tables are not able to change or influence the routing table.
Figure 11 shows a
how a router can dynamically forward packets toward their destination. This
diagram shows that a router contains a routing table (database) that dynamically
changes. This diagram shows a router with address 100 is connected to two other
routers with addresses 800 and 900. Each of these routers periodically exchanges
information allowing them to build routing tables that allow them to forward
packets they receive. This diagram shows that when router 100 receives a packet
for a device number 952, it will forward the packet to router 900. Router 900
will then receive that packet and forward it on to another router that will help
that packet reach its destination.
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