CCNA 2 Week 9 Router Troubleshooting

Copyright © 2005 University of Bolton Topics Routing Table Overview Network Testing Troubleshooting Router Issues

Copyright © 2005 University of Bolton show ip route Fundamental command for understanding routers' behaviour Optional specific commands: connected, rip, igrp, static or address Routers drop any packet without a destination in the table

Copyright © 2005 University of Bolton Gateway of last resort Desirable to define a default option rather than have to consider routing table entries for all options ip default-network net-address –Dynamic routing option – unmatched packets sent via –Routes towards flagged network are default candidates ip route gateway|interface –Static option on a particular router to define

Copyright © 2005 University of Bolton Route Source and Destination Path determination occurs at network layer based on available information: –Supplied by administrator –Learned from traffic conditions Network layer is best-effort end-to-end delivery Routing table used to determine outbound interface

Copyright © 2005 University of Bolton Routing Metrics Can be calculated using constant (bandwidth, delay) or variable (load, reliability) properties IGRP uses all four in a complex equation using 5 parameters K1-K5 to tune metric By default, K1 and K3 are 1, others 0

Copyright © 2005 University of Bolton show ip route Can use as diagnostic: –show ip route –responds with best path from routing table Decisions based on administrative distance and metric Best understood by lab work

Copyright © 2005 University of Bolton Unequal load balancing Routing protocols typically allow load balancing between routes with the same metric IGRP also allows balancing where metrics are different Use variance command plus a value between 1 and 128 Will load balance with routes whose metric is less than n times greater than the best metric

Copyright © 2005 University of Bolton Structured approach

Copyright © 2005 University of Bolton Problems, problems, problems At layer 1 – physical –Cable errors: broken, wrong type, wrong port –Transceiver problems, port switched off At layer 2 – data link –Incorrect configuration of interface –Wrong encapsulation At layer 3 – network –Protocol errors: wrong ip, netmask –Routing not enabled or wrong protocol used

Copyright © 2005 University of Bolton Layer 1 checks Best indicator are lights on the interface Check cable types – try alternatives Use cable tester to verify cables, jacks and wiring

Copyright © 2005 University of Bolton Layer 3 using ping Tests end-to-end connectivity using ICMP echo Tells us whether a connection was successful or not and reports average connection times Typing ping and then on a Cisco router presents user with extensive range of options

Copyright © 2005 University of Bolton Layer 7 using telnet The most complete test mechanism available™ Failure can indicate lower layer issues to check Other higher layer factors may include: –Lack of reverse DNS lookup for DHCP addresses –Telnet server being on another port (default 23) –Security features such as firewalls

Router Issues

Copyright © 2005 University of Bolton show interfaces int Reports status of interface at both layer 1 and 2 show interface s0/0 serial 0/0 is up, line protocol is up indicates carrier detect (l1) and keep alives (l2) Output shows carrier transitions and error rates, both indicative of problems worth investigating

Copyright © 2005 University of Bolton Common layer 1 errors Increasing error count can indicate –Faulty telephone company equipment –Noisy serial line –Incorrect cable or cable length –Damaged cable or connection –Defective CSU or DSU –Defective router hardware

Copyright © 2005 University of Bolton Common layer 2 issues Working serial connections send periodic keep- alive messages. 3 in a row missed line protocol = down Possible problems: –No keepalives –No clock rate –Mismatch in encapsulation type

Copyright © 2005 University of Bolton Using CDP Protocol used to show directly connected neighbours Enable interface using no shutdown and no other configuration Use show cdp neighbors –Missing neighbours indicates layer 1 problems

Copyright © 2005 University of Bolton Using Traceroute Sends series of UDP datagrams to destination Increasing TTL on packets – as each times out a ICMP error sent back (Time Exceeded Message) Must reach destination in less than 30 steps

Copyright © 2005 University of Bolton Troubleshooting routing show ip route –indicates whether a route to destination is known show ip protocols –details of configuration such as timers, outbound interfaces etc.

Copyright © 2005 University of Bolton Using show controllers Query settings of controller circuitry using show controllers interface Reports various information including cable types

Copyright © 2005 University of Bolton Using debug IOS provides extensive debug options to report events to the administrator (normally via console) e.g. debug ip eigrp –Would report all protocol related events such as LSAs Avoid excessive use – can slow router Turn off using no debug whatever