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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-

The RIP protocol is a dynamic, distance-vector routing protocol used in local and wide area networks. This is an implementation of single routing metric (hop count) which measures the best route distance between the source and a destination network RIP. RIP is an interior gateway protocol (IGP) which means that it performs routing within a single autonomous system (AS). RFC1058 and RFC1723 are two formal documents that define this protocol. RIP uses UDP port 520 for route updates and sending messages and is normally sent as a broadcast.

The fundamental operation metric is based solely on the calculations of the number of hops it takes to the destination network. For example, a router is defined to be one hop from a directly connected network; therefore the number of hops along a path from a given source to a given destination is represented as the number of networks that a datagram has encountered along that specified path. RIP has a maximum distance of 15 hops; a hop count of 16 or more is defined as infinity as it is considered unreachable by RIP, hence it is only suitable for small networks. This fact is used by RIP to prevent routing loops.

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The two RIP router types in which participants are divided are activate gateways and passive host. Active participants broadcast their routes to others every 30 seconds whereas passive participants listen and update their routine tables relying on other routers advertisements as they do not advertise. In terms of RIP operations, initially routers are configured with directly connected network addresses, whereby the packets are being routed by the classful routing protocol and are interpreted according to the subnet masks locally configured on the routers interfaces.

Therefore all subnet masks within a major, class-level network must be consistent. RIP broadcast request retrieves RIP information on each RIP-enabled interface whereas the RIP table received is compared to its own table causing the routers to add new entries and update existing destinations. Consequently, a shorter path is advertised meaning the cost will be lower (hop count). Broadcast response packets are sent periodically. When the network tropology changes RIP routing updates, numerous timers are used to enhance the performance by faster convergence.

How long to wait to remove a route after it has timed out. There are two types of RIP request: “complete routing table request” and “specified entries request”. Each RIP routing table entry includes the destination IP Address, metric, next hop, advertising router and timeout in seconds. The basic steps for how the router table handles the update is by firstly checking the validity of the update as if invalid the updated is ignored. Next the corresponding destination is searched.

If the address and destination does not exist in the table then it is updated to the lowest cost. OSPF Open Shortest Path First (OSPF) is considered to be a more enhanced method than RIP and is used within larger system networks. OSPF is a link state protocol whereby a router does a routine check on the condition of each of its connections (links) and reports the information to its neighbours. In addition, it contains whole knowledge of topology which permits routers to calculate routes that satisfy the particular criteria.

OSPF topology determines the routing table presented to the Internet Layer and so makes routing decisions based solely on the destination IP address found in IP packets. OSPF was especially designed to support variable-length subnet masking (VLSM) or Classless Inter-Domain Routing (CIDR) addressing models. With regards to OSFP protocol operations, each router initially exchanges on all OSPF enabled interfaces “hello messages” periodically on each link in order to establish and test neighbour reachability.

Adjacencies form in-between some neighbours that are point to point connected together hence, not all routers can be adjacent. Once this connection is established, virtual network topology information such as Node ID, Sequence number, Router/ DR/ Network state are shared with linked adjacent neighbours in means of a packed Link State Advertisement (LSA). Information is added by nodes to the LS database and then state information is forwarded to direct neighbours, who then follow this procedure. These are only sent once and are updated every 30 seconds if there is any change by sending the complete routing table.

Secondly, each router sends the 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.


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