CCNA2

1. CCNA2 Module 9 M. Menelaou

2. One of the primary functions of a router is to determine the best path to a given destination. • A router learns paths, also called routes, from an administrator's configuration or from other routers by way of routing protocols. • Routers store this routing information in routing tables using on-board dynamic random access memory (DRAM). • A routing table contains a list of the best available routes M. Menelaou

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5. It is not feasible, or even desirable, for a router to maintain routes to every possible destination • Instead, routers keep a default route, or a gateway of last resort. Default routes are used when the router is unable to match a destination network with a more specific entry in the routing table M. Menelaou

6. Default routing begins with the administrator • Before routers can dynamically exchange information, an administrator must configure at least one router with a default route • ip default-networkor ip route 0.0.0.0 0.0.0.0 M. Menelaou

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9. After configuring a default route or default network, the command show ip route will show the following: • Gateway of last resort is 172.16.1.2 to network 0.0.0.0 M. Menelaou

10. at each interface, as the packet moves across the network, the routing table is examined and the router determines the next hop. • The packet is then forwarded using the MAC address of that next hop. • The IP source and destination headers do not change, at any time. M. Menelaou

11. The Layer 3 address is used to route the packet from the source network to the destination network. • The source and destination IP addresses remain the same. The MAC address changes at each hop or router • A data-link layer address is necessary because delivery within the network is determined by the address in the Layer 2 frame header, not the Layer 3 packet header. M. Menelaou

12. The administrative distance of the route is the key information that the router uses in deciding which is the best path to a particular destination. • The administrative distance is a number that measures the trustworthiness of the source of the route information. The lower the administrative distance, the more trustworthy the source M. Menelaou

13. Different routing protocols have different default administrative distances • A route is not installed in the routing table if the administrative distance from another source is lower M. Menelaou

14. Routing protocols use metrics to determine the best route to a destination. The metric is a value that measures the desirability of a route. • Some routing protocols use only one factor to calculate a metric M. Menelaou

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16. IGRP calculates the metric by adding the weighted values of different characteristics of the link to the network in question M. Menelaou

17. In the following example the values bandwidth, bandwidth divided by load, and delay, are weighted with the constants K1, K2, and K3. • Metric=K1 *Bandwidth + (K2 * Bandwidth)/256-load) + K3 * Delay • The default constant values are K1=K3=1 and K2=K4=K5=0 so: • Metric=Bandwidth + Delay M. Menelaou

18. Some routing protocols support multiple paths to the same destination. Unlike single path algorithms, these multi-path algorithms permit traffic over multiple lines, provide better throughput, and are more reliable M. Menelaou

19. The commands telnet and ping are two important commands that are used for testing a network. M. Menelaou

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21. Layer 1 errors can include: • Broken cables • Disconnected cables • Cables connected to the wrong ports • Intermittent cable connection • Wrong cables used for the task at hand (must use rollovers, crossover cables, and straight-through cables correctly) • Transceiver problems • DCE cable problems • DTE cable problems • Devices turned off M. Menelaou

22. Layer 2 errors can include: • Improperly configured serial interfaces • Improperly configured Ethernet interfaces • Improper encapsulation set (HDLC is default for serial interfaces) • Improper clockrate settings on serial interfaces • Network interface card (NIC) problems M. Menelaou

23. Layer 3 errors can include: • Routing protocol not enabled • Wrong routing protocol enabled • Incorrect IP addresses • Incorrect subnet masks M. Menelaou

24. The ping command is used at Layer 3 to test connectivity. • At Layer 7 the telnet command may be used to verify the application layer software between source and destination stations. M. Menelaou

25. Ping • The ping command can be invoked from both user EXEC mode and privileged EXEC mode. • The ping command can be used to confirm basic network connectivity • To use extended ping, type ping at the command line, then press the Enter key without entering an IP address M. Menelaou

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28. Telnet • The telnet utility is a virtual terminal protocol that is part of the TCP/IP protocol suite. It allows verification of the application layer software between source and destination stations. • This is the most complete test mechanism available. The telnet utility is normally used to connect remote devices, to gather information, and to run programs. M. Menelaou

29. Troubleshooting Router • The Cisco IOS contains a rich set of commands for troubleshooting. Among the more widely used are the show commands • show interfaces • show interfaces FastEthernet • show interfaces serial0/0. • show version M. Menelaou

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31. This is determined by whether keepalives are successfully received. M. Menelaou

32. If the show interfaces output shows the last clearing of the counters as never, use the show version command to find out how long the router has been functional. • Use the clear counters command to reset the counters to zero • These counters should always be cleared after an interface problem has been corrected M. Menelaou

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34. The show interfaces command is perhaps the single most important tool to discover Layer 1 and Layer 2 problems with the router. • The first parameter (line) refers to the physical layer M. Menelaou

35. The second parameter (protocol) indicates whether the IOS processes that control the line protocol consider the interface usable • This is determined by whether keepalives are successfully received. • Keepalives are defined as messages sent by one network device to inform another network device that the virtual circuit between the two is still active M. Menelaou

36. If the interface is up and the line protocol is down, a Layer 2 problem exists. Among the possible causes are: • No keepalives • No clock rate • Mismatch in encapsulation type M. Menelaou

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38. Troubleshooting using show cdp • The output from the show cdp neighbors command displays information about directly connected neighbors M. Menelaou

39. Troubleshooting using traceroute • The traceroute command is used to discover the routes that packets take when traveling to their destination. • Traceroute can also be used to help test the network layer (Layer 3) on a hop-by-hop basis and provide performance benchmarks. M. Menelaou

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41. Troubleshooting using show ip route • The show ip route command is perhaps the single most important command for troubleshooting routing issues • This command displays the contents of the IP routing table. • The output from the show ip route command shows the entries for all known networks and subnetworks, and how that information was learned M. Menelaou

42. Troubleshooting using show ip protocols • The show ip protocols command displays values about IP routing protocol information on the entire router. • This command can be used to confirm which protocols are configured, which networks are being advertised, which interfaces are sending updates, and the sources of routing updates M. Menelaou

43. Troubleshooting using show controllers serial • By examining the show controllers serial command output, the type of cable that the controller detects can be determined. This is useful for finding a serial interface with no cable, the wrong type of cable, or a defective cable. M. Menelaou

44. Debug • The debug commands assist in the isolation of protocol and configuration problems • The debug command is used to display dynamic data and events • The dynamic output of the debug command comes at a performance cost, producing high processor overhead that may disrupt normal router operation M. Menelaou

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49. RIP routing is configured Interfaces sending and receiving RIP updates Router is advertising the correct networks Verifying RIP Configuration Dublin# show ip protocols M. Menelaou

50. Layer 1 Troubleshooting Physical layer (line status) Up: has Layer 1 connectivity Down: L1 problem Administratively down: disabled Bad line, bad hardware Faulty hardware, cable or noise Line interruptions, faulty hardware • Check power. • Check the NIC or interface’s ‘Link Light’. • Make sure interface cards are inserted correctly. • Ensure appropriate, known-good cable is correctly attached (both ends). • Verify that switch ports are in the correct collision domain (VLAN), and have appropriate options set (spanning tree). • Check transceiver is attached and functional. BHM# show interface s0 Serial0 is up, line protocol is up < output omitted > Last clearing of "show interface" counters never < output omitted > Received 73 broadcasts, 0 runts, 0 giants, 0 throttles 0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort 135 packets output, 7361 bytes, 0 underruns 0 output errors, 0 collisions, 37 interface resets 0 output buffer failures, 0 output buffers swapped out 54 carrier transitions To reset: BHM# clear counters M. Menelaou