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CiscoRoutingTable2_Lookup - The Routing Table Part 2 The...

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The Routing Table Part 2 – The Routing Table Lookup Process By Rick Graziani Cisco Networking Academy [email protected] Updated: Oct. 10, 2002 This document is the second of two parts dealing with the routing table. Part I discussed the structure of the routing table, and how routes are created. Part II will discuss how the routing table lookup process finds the “best” route within the routing table. What happens when a router receives an IP packet, examines the IP destination address and looks that address up in the routing table? How does the router decide which route in the routing table is the best match? What affect does the subnet mask have on the routing table? How does the router decide whether or not to use a supernet or default route if there is not a better match? We will answer these questions and more in this document. The network we will be using is a simple three router network. RouterA and Router B share the common major network RouterB and RouterC are connected by the network. You will notice that RouterC also has a subnet which is disconnected, or discontiguous, from the rest of the network. Router B Router C Router A s0 s0 s0 s1 fa0 fa0 fa0 .1 .1 .2 .1 .1 .1 .2 Most of this information is best explained by using examples. So, don’t worry if you do not understand the information before you have seen an example, as there is an example for every topic. One last note before we begin. As I mentioned previously in Part I, in order to keep this document simple and brief, I have left out some of the detail. For those of you who are interested in reading more about this subject and the inner-workings of the Cisco IOS as it pertains to routing, I highly recommend the book, Cisco IP Routing, by Alex Zinin (ISBN 0-201-60473-6) . However, this is not a book for beginners. It does contain some pseudocode, which can be skipped without missing any of the content. The Setup We will begin by assuming the following configurations have already been done to the three routers. If you are using the document as a lab, configure the routers with these configurations. You will notice that we are using RIPv1, a classful, distance-vector routing protocol. 1 The DCE cable is attached the serial interfaces of RouterA and RouterC. The command exec-timeout 0 0 is an optional command that keeps the router from exiting privileged mode when the idle timer expires. The other optional command is logging synchronous . This command eliminates the debug and other output from becoming intermixed with the router prompt and input commands.
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RouterA hostname RouterA ! interface FastEthernet0 ip address ! interface Serial0 ip address clockrate 64000 ! router rip network ! ip classless !
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CiscoRoutingTable2_Lookup - The Routing Table Part 2 The...

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