Specifying a Next Hop IP Address for Static Routes. Configuring a Gateway of Last Resort Using IP Commands. Route Selection in Cisco Routers. Introduction One of the intriguing aspects of Cisco routers, especially for those new to routing, is how the router chooses which route is the best among those presented by routing protocols, manual configuration, and various other means.
While route selection is much simpler than you might imagine, to understand it completely requires some knowledge about the way Cisco routers work. Prerequisites Requirements There are no specific prerequisites for this document. Components Used This document is not restricted to specific software and hardware versions. Conventions For more information on document conventions, see the Cisco Technical Tips Conventions. Processes Involved There are three processes involved in building and maintaining the routing table in a Cisco router: Let's examine the interaction between the routing protocols and the routing table to understand how the routing table is built. Building the Routing Table The main considerations while building the routing table are: Redistributing Routing Protocols. Introduction The use of a routing protocol to advertise routes that are learned by some other means, such as by another routing protocol, static routes, or directly connected routes, is called redistribution.
While running a single routing protocol throughout your entire IP internetwork is desirable, multi-protocol routing is common for a number of reasons, such as company mergers, multiple departments managed by multiple network administrators, and multi-vendor environments. Running different routing protocols is often part of a network design.
In any case, having a multiple protocol environment makes redistribution a necessity. Differences in routing protocol characteristics, such as metrics, administrative distance, classful and classless capabilities can effect redistribution. Prerequisites Requirements. Redistributing Between Classful and Classless Protocols: EIGRP or OSPF into RIP or IGRP. Introduction This document explains two common problems with redistributing routes between RIP and OSPF or IGRP and EIGRP.
RIP and IGRP will not advertise routes out an interface if those routes are on the same major network, but have a different mask than that particular interface. For more information on RIP and IGRP updates, refer to Behavior of RIP and IGRP When Sending and Receiving Updates. Prerequisites Requirements. Problems with Running OSPF in NBMA and Broadcast Mode over Frame Relay. How Does OSPF Generate Default Routes? Introduction The way that Open Shortest Path First (OSPF) generates and advertises default routes (0.0.0.0) varies depending on the type of area the default route is being injected into.
In this document, we cover normal areas, stub/totally stub areas and not-so-stubby areas (NSSAs). Before You Begin Conventions For more information on document conventions, refer to the Cisco Technical Tips Conventions. OSPF show Commands Respond Slowly [IP Routing. Redistributing Connected Networks into OSPF. Introduction This document describes the behavior of redistributing connected routes into Open Shortest Path First (OSPF).
There are two behaviors dependent on which version of Cisco IOS® software you are running. Prerequisites Requirements Readers of this document should have knowledge of these topics: Common Routing Problem with OSPF Forwarding Address. Introduction This document describes the concepts and the problem associated with the Open Shortest Path First (OSPF) forwarding address.
Refer to Why Are Some OSPF Routes in the Database but Not the Routing Table? Why Are Some OSPF Routes in the Database but Not in the Routing Table? [IP Routing. IP Routing Troubleshooting TechNotes. The "%TUN-5-RECURDOWN" Error Message and Flapping EIGRP/OSPF/BGP Neighbors Over a GRE Tunnel. Troubleshooting TCP/IP. If a network administrator has chosen to use 8 bits of subnetting, the third octet of a Class B IP address provides the subnet number. For example, address 172.16.1.0 refers to network 172.16, subnet 1; address 172.16.2.0 refers to network 172.16, subnet 2; and so on. In today's world, the difference between subnet bits and the natural mask has become blurred, and you will often see only a prefix length that specifies the length of the entire mask (natural mask plus subnet bits).
It is still important to understand the difference between the natural network mask, which is determined by the network class, and the subnet mask, because routers sometimes make assumptions based on the natural mask of an address. For example, the natural mask of 10.1.1.1/24 is 8 bits because this is a class A network, even though the subnet mask is 24 bits. Subnet masks can be expressed in two forms: prefix length (as in /24), or dotted-decimal notation (As in 255.255.255.0). How Does Load Balancing Work? Introduction Load balancing is a standard functionality of the Cisco IOS® router software, and is available across all router platforms.
It is inherent to the forwarding process in the router and is automatically activated if the routing table has multiple paths to a destination. It is based on standard routing protocols, such as Routing Information Protocol (RIP), RIPv2, Enhanced Interior Gateway Routing Protocol (EIGRP), Open Shortest Path First (OSPF), and Interior Gateway Routing Protocol (IGRP), or derived from statically configured routes and packet forwarding mechanisms. It allows a router to use multiple paths to a destination when forwarding packets. Prerequisites Requirements.