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4.2.4.2 Routing algorithms, RIP
The rate of information processing and its trustworthiness depend on the routing algorithm. But
more complicated and high-speed algorithms imply high requirements to the router’s capacity.
Static routing algorithms are the simplest ones. The network administrator establishes routing
tables, and they do not change until the network administrator changes them. Algorithms of static
routers are simple to design and they work well in simple networks with low traffic.
Dynamic routing algorithms are more complicated ones. They adjust in real time to network
changes. They do this by analyzing incoming routing update messages. If the router receives a
message about a network change, it makes updates it’s routing table and sends out this
information to all the nodes.
The Routing Information Protocol (RIP), implemented in MDS92xxx-10BT modems is a dynamic
routing protocol.
RIP routing tables contain information about packet destination, next hop, and hop counts
(metrics). The routing table can also contain other information such as timers.
Destination Next hop Distance Timers Flags
Network A Router 1 3 t1, t2, t3 x,y
Network B Router 2 5 t1, t2, t3 x,y
Network C Router 1 2 t1, t2, t3 x,y
RIP supports only optimal routes to destinations. If new information provides a better route, this
information updates the old one. Changes in the network topology can cause changes in the
routes, resulting, for example, in creation of better routes to a definite destination. If the network
topology changes, these changes are reflected in updating messages. For example, when a
router finds a failure of one of the links or another router, it recalculates its own routes and sends
out routing updates. Each router that receives routing update messages, includes changes to its
tables and sends them out.
4.2.4.3 Internet Access through LANs, NAT
The Network Address Translation technology allows one to solve to main problems the Internet
faces now. This is a restriction of the address space of IP and routing scaling.
If necessary to get an Internet access, when the number of network nodes connected to the
Internet provider is bigger that the number of IP addresses, NAT allows private IP networks,
using unregistered addresses, to get an access to Internet resources. NAT functions are
configured on a border router, dividing Intranet and Internet networks.
If necessary to change internal address system, instead of a complete change of all the
addresses (and this is quite a pain-taking process), NAT allows translating them according to the
new address plan.
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