Describe the Operation of Rip and Ospf and Discuss Mechanisms That Can Be Used to Improve the Reconvergence Time of These Interior Gateway Routing Protocols. What Are the Consequences of Least-Cost Routing in a Single-

Links: State Advertisements (LSA’s) over all adjacencies which describe the routers links, interface and state. All the routers receive the LSA’s and are then added to their link state database. The information retrieved is then passed along to its neighbours thus identically linking the state database which is constructed by each router. Here-after SPF algorithm is implemented on the database to obtain a SPF tree. Ultimately, output is installed into the routing table from the SPF tree. RIP vs. OSPF The only common link between RIP and OSPF is that both are dynamic IGP protocols. Nevertheless, rapid expansions of networks in recent times are causing RIP to slowly die out due to the fact that RIP has certain limitations that could cause problems in large networks. RIP has a limit of 15 hops. A RIP network that spans more than 15 hops (15 routers) is considered unreachable and therefore not scalable in broadcast mediums. On the other hand OSPF is not impervious to size nor is it affected as it employs a hierarchical topology in which it breaks a single large AS (Autonomous System) into smaller ones. These are called “areas” and are configured with a default route that summarises all routes outside an area of AS. As a result, OSPF is much scalable to very large internetworks. RIP cannot handle Variable Length Subnet Masks (VLSM) and CIDR compared to OSPF. This is considered one of the major flaws in implementing RIP given the shortage of IP addresses and the flexibility VLSM gives in the efficient assignment of IP addresses. In contrast to OSPF, RIP is less secure and it has poor robustness as RIP only stores one path to a destination. Furthermore, RIP converges slower than OSPF. In large networks convergence gets to be in the order of minutes, however RIP routers will go through a period of a hold-down and garbage collection and will slowly time-out information that has not been received recently. This is inappropriate in large environments and could cause routing inconsistencies. Moreover, RIP employs periodic broadcasts of the updates in the network (full routing table) which consumes a large amount of bandwidth. This is a major problem with large networks especially on slow links and WAN clouds. In comparison, OSPF have event-triggered updates thus giving a much faster convergence time compare to IP. In terms of the routing tables, RIP has no concept of network delays and link costs because routing decisions are based on the calculation to obtain hop counts. Therefore, the path with the lowest hop count to the destination is always preferred even if the longer path has a better aggregate link bandwidth and slower delays. Alternatively, OSPF routers have a common view of the whole network topology whereby the best route is based on the shortest path of each router. This is implemented by linking states passing through each router in the network and then by taking into consideration the distance and state of the hops. To conclude, by comparing the two protocols it is evident that RIP is better suited in smaller networks even though they tend to be flat networks as there is no concept of areas or boundaries. As a result of implementing classless routing and the intelligent use of aggregation and summarization, OSPF has excelled whereas RIP networks seem to have fallen behind. Hence it remains clear that OSPF is more suitable for larger networks.