ccna 2 week 9 router troubleshooting. copyright © 2005 university of bolton topics routing table...
TRANSCRIPT
CCNA 2 Week 9
Router Troubleshooting
Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Topics
Routing Table Overview
Network Testing
Troubleshooting Router Issues
Copyright © 2005 University of BoltonCopyright © 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 BoltonCopyright © 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 0.0.0.0 0.0.0.0 gateway|interface– Static option on a particular router to define
Copyright © 2005 University of BoltonCopyright © 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 BoltonCopyright © 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
45
3256
21
krel
kdelayk
load
bwkbwkMetric
Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
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
Copyright © 2005 University of BoltonCopyright © 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 BoltonCopyright © 2005 University of Bolton
Structured approach
Copyright © 2005 University of BoltonCopyright © 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 BoltonCopyright © 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 BoltonCopyright © 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 <enter> on a Cisco router presents user with extensive range of options
Copyright © 2005 University of BoltonCopyright © 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 BoltonCopyright © 2005 University of Bolton
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
Copyright © 2005 University of BoltonCopyright © 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 BoltonCopyright © 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 BoltonCopyright © 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 BoltonCopyright © 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 BoltonCopyright © 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 BoltonCopyright © 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 BoltonCopyright © 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