CCNA 2 Week 9 Router Troubleshooting. Copyright © 2005 University of Bolton Topics Routing Table Overview Network Testing Troubleshooting Router Issues

CCNA 2 Week 9

Router Troubleshooting

Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton

Topics

Routing Table Overview

Network Testing

Troubleshooting Router Issues


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


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 0.0.0.0 0.0.0.0 gateway|interface– Static option on a particular router to define


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


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

45

3256

21

krel

kdelayk

load

bwkbwkMetric


show ip route

Can use as diagnostic:– show ip route 192.168.252.0– responds with best path from routing table

Decisions based on administrative distance and metric

Best understood by lab work


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


Structured approach


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


Layer 1 checks

Best indicator are lights on the interface

Check cable types – try alternatives

Use cable tester to verify cables, jacks and wiring


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 <enter> on a Cisco router presents user with extensive range of options


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


show interfaces int

Reports status of interface at both layer 1 and 2

show interface s0/0serial 0/0 is up, line protocol is upindicates carrier detect (l1) and keep alives (l2)

Output shows carrier transitions and error rates, both indicative of problems worth investigating


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


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


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


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


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.


Using show controllers

Query settings of controller circuitry using show controllers interface

Reports various information including cable types


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

Documents

CCNA 2 Week 9 Router Troubleshooting. Copyright © 2005 University of Bolton Topics Routing Table Overview Network Testing Troubleshooting Router Issues