The routing processes are each assigned an administrative distance, which is used to decide which route to install. Now, all four of these processes have learned of various routes to the 192.168.24.0/24 network, and each has chosen its best path to that network through its internal metrics and processes.Įach of these four processes attempts to install their route toward 192.168.24.0/24 into the routing table. If this route is not the route with the best administrative distance, then the route is rejected.įor example, assume a router runs four routing processes: EIGRP, OSPF, RIP, and IGRP. If this path has the lowest administrative distance to this destination (when compared to the other routes in the table), it is installed in the routing table. The router decides whether or not to install the routes presented by the routing processes based on the administrative distance of the route in question. For instance, if EIGRP learns of a path toward 10.1.1.0/24, and decides this particular path is the best EIGRP path to this destination, it tries to install the path it has learned into the routing table. Each routing protocol uses a different metric.Īs each routing process receives updates and other information, it chooses the best path to any given destination and attempts to install this path into the routing table. Metrics- This is a measure used by the routing protocol to calculate the best path to a given destination, if it learns multiple paths to the same destination. If a router learns about a destination from more than one routing protocol, the administrative distance is compared and the preference is given to the routes with lower administrative distance. The main considerations when you build the routing table are:Īdministrative distance- This is the measure of trustworthiness of the source of the route. You need to examine the interaction between the routing protocols and the routing table to understand how the routing table is built. The forwarding process, which requests information from the routing table to make a packet forwarding decision. The routing table itself, which accepts information from the routing processes and also replies to requests for information from the forwarding process. Various routing processes, which actually run a network (or routing) protocol, such as Enhanced Interior Gateway Routing Protocol (EIGRP), Border Gateway Protocol (BGP), Intermediate System-to-Intermediate System (IS-IS), and Open Shortest Path First (OSPF). There are three processes involved to build and maintain the routing table in a Cisco router: Route selection requires some knowledge about the way Cisco routers work. One aspect of Cisco routers is how the router chooses the best route among those presented by protocols, manual configuration, and various other means. Conventionsįor more information on document conventions, see the Cisco Technical Tips Conventions. If your network is live, ensure that you understand the potential impact of any command. All of the devices used in this document started with a cleared (default) configuration. The information in this document was created from the devices in a specific lab environment. This document is not restricted to specific software and hardware versions. There are no specific prerequisites for this document. This document describes how routers work, are configured, and how to select a route for them.
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