Network Topologies

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a. Explain the differences between RIP and OSPF?

RIP
RIP is an Interior Gateway Protocol (originally defined by RFC 1058 ) is the most widely accepted routing protocol. It is also known by the name of the Unix daemon program "routed", which was originally designed at U.C. Berkeley to provide consistent routing and reach-ability information among machines on a local area network. RIP's popularity is not necessarily based on its technical merits, but probably results because U.C. Berkeley distributed "routed" along with their popular 4.x BSD UNIX systems. Thus, many Internet sites adopted and installed "routed" and started using RIP without even considering its technical merits and limitations. Once installed and running, it became the basis for local routing.
RIP is straightforward: it arranges to have routers to broadcast their entire current routing database periodically, typically every 30 seconds. This message lists each destination along with a ``distance'' to that destination measured in number of router hops. RIP is also known as a distance-vector routing algorithm, which means that there is a distance (a cost) and a vector (a direction) for each destination (the vector just shows the name of the neighboring router, not the entire path).
As a distance-vector based algorithm, RIP works fine for small, stable high-speed networks. Instead of passing along the status of the links to the networks, the router tells its neighbors about the entire world, where the ``best link'' is precomputed, not necessarily the fastest one. Here, ``best'' means a route with the least number of gateway hops, relative to the active routing table.
In times of network instability--because every router is broadcasting his entire routing table--it takes awhile for those states to converge into a common view of the network topology. Specifically, RIP does not address well these major types of problems:
➢ There is no protection from routing loops within RIP based networks. Therefore, the implementation must trust all network participants to prevent such loops.
➢ RIP uses a hop count of 15 do denote infinity, which makes it unsuitable for large networks.
➢ RIP has the so called slow convergency or count-to-infinity problem, in which inconsistencies arise because routing update message propagate slowly across the network. Particularly, in large networks (or networks with slow links), some routers may still advertise a route that has vanished. (That, by the way, was one of the reasons 15 was chosen as the value of infinity to limit the slow convergency).
Slow convergency can be addressed by a technique called hold down, which forces the router to ignore information about a network for a fixed period of time (typically, 60 seconds). The idea is to wait long enough so that all machines receive the bad news of the vanished link and do not mistakenly accept outdated information. The disadvantage of this technique is that incorrect routes and routing loops will be preserved for the duration of the hold down, even when a valid alternate path is available.
Another, more popular approach to address the slow convergency problem is a technique known as split-horizon update. This technique is widely used by router vendors. With split horizon, a router records the interface over which it received a particular route and does not propagate its higher-cost route back over the same interface. However, split horizon does not resolve the slow convergency problem for all topologies. It also introduces a problem for a frame relay network that would not permit full-meshed connectivity for partially meshed networks.
RIP allows the use of subnet masks with static information defined in the routers. RIP has no provision to exchange subnet mask information between routers. The routers in your network should use the same subnet masks and RIP to make IP datagrams travel from source to destination within your network. This is another technical limitation of RIP. For example, with frame relay, the entire frame relay network can be assigned one subnet, but as was mentioned above, because of the split horizon technique it would not allow connectivity between two remote sites via a centrally based router, and require explicit definitions of additional routes for all remote sites.
If the subnet mask is not constant i n a RIP based network, it becomes impossible for the routers to distinguish between network part and host part because RIP cannot dynamically update or change the subnet mask. Hence one mask should be used throughout the sub-network. If a variable subnet mask is needed, static routes or more advanced routing protocols, for instance, OSPF, have to be used.

OSPF
OSPF, as defined by RFC 1131, is a link-state algorithm. In contrast to a distance-vector algorithm, where a router ``tells all neighbors about the world,'' link-state routers ``tell the world about the neighbors.'' OSPF specifies a class of messages called link-state ...