Distance vector method of updating routing table information examples
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Distance-vector routing protocol
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Discover its neighbors and build its neighbor table Measure the cost delay, bandwidth, etc to each of its neighbors Construct and send a routing update telling all it has learned to all routers in the network Apply updatinng Dijkstra algorithm to construct the shortest path to all possible destinations Below you'll find more details on the four step process. Step 1: Within the replies, network addresses of the routers are attached and are used updzting the HELLO initiator to build up its neighbor table. Step 2: Measuring Link Cost A set of tests is performed on each router to measure the cost to each of its neighbors.
The cost could be a measure of the end-to-end delay, throughput, or a combination of these metrics. How these tests are performed is out of the scope of this article. The important thing to know is that each link state enabled router has to somehow possess an estimate of the cost for each of its links. Building and Distributing Link State Packets Each router builds up a packet containing its neighbors and the corresponding link costs to these neighbors. At the beginning of the packet, each router adds its identity along with a sequence number and an age parameter, the latter being used to ensure no packet will wander around for an indefinite period of time.
After the construction process, the packet is flooded in the network.
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Based on the same subnet diagram presented edamples the equivalent link state packets for each of the routers are presented below. Random link cost values have been assumed. Also for simplicity, sequence and age Distacne are omitted. Step 4: And like the sign at the fork in the trail, they are vulnerable to accidental or intentional misdirection. Following are some of the difficulties and refinements associated with distance vector algorithms. Route Invalidation Timers Now that the internetwork in Figure 4. If network But what if, instead of Routers Vevtor, B, and C still have entries in their route tables about This problem is handled by setting a route invalidation timer for each entry in the route table.
For example, when router C first hears about At every regularly scheduled update from router D, C discards the update's already-known information about If router D goes down, C will no longer hear updates about The timer will expire, C will flag the route as unreachable and will pass the information along in the next update. Typical periods for route timeouts range from three to six update periods. A router would not want to invalidate a route after a single update has been missed, because this event may be the result of a corrupted or lost packet or some sort of network delay. At the same time, if the period is too long, reconvergence will be excessively slow.
Split Horizon According to the distance vector algorithm as it has been described so far, at every update period each router broadcasts its entire route table to every neighbor. But is this really necessary?
Every network known by router A in Figure 4. Common sense suggests that for router A to broadcast the updatnig it has learned from router B back to Distznce B is a waste of resources. Obviously, B already knows about those networks. A route pointing back to the router from which packets were received is called a reverse route. Split horizon is a technique for preventing reverse routes between two routers. Besides not wasting resources, there is a more important reason for not sending reachability information back to the router from which the information was learned.
Look yet again at the converged internetwork of Figure 4.
Updating method table vector of Distance examples routing information
Router D will detect updatihg failure, flag the network as unreachable, mwthod pass the information along to router C at the next update interval. However, before D's update timer triggers an update, something unexpected happens. C's update arrives, claiming that it can reach Remember the road sign analogy? Router D has no way of knowing that C is not advertising a legitimate next-best path. It will increment the hop count and make an entry into its route table indicating that Now a packet with a destination address of C consults its route table and forwards the packet to D.
D consults its route table and forwards the packet to C, C forwards it back to D, ad infinitum. A routing loop has occurred. Implementing split horizon prevents the possibility of such a routing loop. There are two categories of split horizon: RIPv1 is not suitable for large networks as it limits the number of hops to This hop limit was introduced to avoid routing loops, but also means that networks that are connected through more than 15 routers are unreachable. BGP is a exterior gateway protocol and therefore implemented on border and exterior routers on the Internet. Routers with BGP implementation determine the shortest path across a network based on a range of factors other than hops.
BGP can also be configured by administrators so that certain routes are preferred or avoided. It was developed by Cisco in the s and was designed to offer better convergence and cause less network traffic between routers than the link-state routing protocol Open Shortest Path First OSPF. Network topology changes[ edit ] Compared to link-state protocolswhich require a router to inform all the nodes in a network of topology changes, distance-vector routing protocols have less computational complexity and message overhead. The core of the count-to-infinity problem is that if A tells B that it has a path somewhere, there is no way for B to know if the path has B as a part of it.
Flooding works in the following way. If not, it stores the LSP. If it already has a copy, it compares the sequence numbers; if the new LSP has a larger sequence number, it is assumed to be the more recent, and that LSP is stored, replacing the old one. Each switch computes its routing table directly from the LSPs it has collected using a realization of Dijkstra's algorithm. Every 10 S the average delay of all packets is computed. The delay is considered to have changed "by a significant amount" whenever the absolute value of the change exceeds a certain thershold.