ccna7

ccna7.Chapter 14 Routers and Routing 347However, what would be the result if Router A had a failure on interface E0 and received an update from Router B indicating that it had an alternate path to 192.168.1.0? Here, the network clearly has connectivity issues. However, with horizontal segmentation enabled, Router B will never advertise 192.168.1.0 to Router A. Therefore, if Router A's interface E0 fails, both Router A and Router B will know that there is no alternate path to that network until Router A's interface E0 returns to normal.Exam Essentials Horizontal segmentation prohibits a router from re-announcing a route from the interface that received it.Solution: Route Suppression and Suppression TimerHorizontal partitioning is used to solve small routing loops. to cope with large routing loops, the distance vectoring protocol uses two mechanisms: route suppression and suppression timers. Route suppression is derived from horizontal partitioning. After a router finds that its directly connected route has failed, it sets its metric to infinity to suppress it. In RIP, the number of hops for such a route is set to 16 (the default maximum is 15), thus indicating that the network is unreachable. When a router advertises a suppressed route to a neighbor, the neighbor sends it back to it in violation of the horizontal split rule, which is called reverse suppression and is intended to ensure that all routers will receive the initial suppressed route update OExam Essentials The metric for suppressed routes is infinity. Reverse suppression lets the router violate the horizontal split rule by advertising suppressed routes out of all interfaces.To allow the router enough time to propagate the suppressed route and to ensure that no routing loops occur during this time, the router implements a suppression timer. The router will freeze the suppressed routes in its routing table until the suppression timer expires, which is typically three times the routing broadcast update interval.The suppressed routes will remain in the routing table until the suppression timer expires. However, if, in the meantime, the router receives a routing update from a neighboring router with a metric equal to or smaller than the suppressed route, the router will break the suppression, remove the suppressed route, and add the new route to the routing table. Alternatively, if the router receives a worse route from a neighboring router, it will consider the route suspicious and possibly part of a routing loop, and thus ignore it. Of course, a route with a worse metric may actually be a valid alternative route, but the suppression timer and route suppression prohibit the use of such routes until the suppression timer expires. During suppression, suppressed routes show up in the routing table as possibly invalid.One of the problems with using the suppression timer is that it results in extremely slow convergence of the distance vector routing protocol - valid alternative routes with worse metric values cannot be used during the 180-second suppression period. As a result, users are unable to connect to the network corresponding to the suppressed route for at least 3 minutes. The suppression timer is used to ensure that the suppressed route remains in the routing table long enough for it to propagate to all routers in the network. A disadvantage of suppression timers is that they prolong the convergence time °Routing suppression and suppression timer example