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Dynamic Routing

Remote networks can also be added to the routing table by using a dynamic routing protocol. In the figure, R1 has automatically learned about the network from R2 through the dynamic routing protocol, RIP (Routing Information Protocol). RIP was one of the first IP routing protocols and will be fully discussed in later chapters.

Note: R1's routing table in the figure shows that R1 has learned about two remote networks: one route that dynamically used RIP and a static route that was configured manually. This is an example of how routing tables can contain routes learned dynamically and configured statically and is not necessarily representative of the best configuration for this network.

Dynamic routing protocols are used by routers to share information about the reachability and status of remote networks. Dynamic routing protocols perform several activities, including:
Network discovery
Updating and maintaining routing tables

Automatic Network Discovery

Network discovery is the ability of a routing protocol to share information about the networks that it knows about with other routers that are also using the same routing protocol. Instead of configuring static routes to remote networks on every router, a dynamic routing protocol allows the routers to automatically learn about these networks from other routers. These networks - and the best path to each network - are added to the router's routing table and denoted as a network learned by a specific dynamic routing protocol.

Maintaining Routing Tables

After the initial network discovery, dynamic routing protocols update and maintain the networks in their routing tables. Dynamic routing protocols not only make a best path determination to various networks, they will also determine a new best path if the initial path becomes unusable (or if the topology changes). For these reasons, dynamic routing protocols have an advantage over static routes. Routers that use dynamic routing protocols automatically share routing information with other routers and compensate for any topology changes without involving the network administrator.

IP Routing Protocols

There are several dynamic routing protocols for IP. Here are some of the more common dynamic routing protocols for routing IP packets:
RIP (Routing Information Protocol)
IGRP (Interior Gateway Routing Protocol)
EIGRP (Enhanced Interior Gateway Routing Protocol)
OSPF (Open Shortest Path First)
IS-IS (Intermediate System-to-Intermediate System)
BGP (Border Gateway Protocol)

Note: RIP (versions 1 and 2), EIGRP, and OSPF are discussed in this course. EIGRP and OSPF are also explained in more detail in CCNP, along with IS-IS and BGP. IGRP is a legacy routing protocol and has been replaced by EIGRP. Both IGRP and EIGRP are Cisco proprietary routing protocols, whereas all other routing protocols listed are standard, non-proprietary protocols.

Once again, remember that in most cases, routers contain a combination of static routes and dynamic routes in the routing tables. Dynamic routing protocols will be discussed in more detail in Chapter 3, "Dynamic Routing Protocols."
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Cindy Dy said...

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