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CCNA 2 v3.1 Module 9

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Basic Router Troubleshooting

CCNA 2, Module 9

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Router#show ip route

• Displays the content of the IP routing table

Entries for all known networks and subnets

Code that indicates how network was learned

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New Routes can be Added

• Statically

Administrator manually defines routes

Routes do not change until administrator manually programs the changes

• Dynamically

Routing protocol exchange routing information

Router independently selects the best path

Routes change automatically as neighboring routers update each other with new information

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Static Routing Advantages

• Lower processor overheads

• Lower memory overheads

• No bandwidth utilization

• Secure operation

• Predictability

Static Routing Disadvantages

• High maintenance configuration

• No adaptability to change

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Dynamic Routing Advantages

• High degree of adaptability

• Low maintenance configuration

Dynamic Routing Disadvantages

• Increased Processor overheads

• Increased memory utilization

• High bandwidth utilization

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Gateway of Last Resort (Default Gateway)

• Used when the router is unable to match a destination network

• keep routing tables as lean as possible

Router can forward packets destined to any Internet host without having to maintain entry for every Internet network

• Configured by administrator• Router(config)#ip default network <network no>

Or Router(config)#ip route 0.0.0.0 0.0.0.0 <next-hop or outgoing port>

• To find out the default route use show ip route

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Route Source and Destination

• Path determination occurs at network layer

• The network layer provides best-effort, end-to-end, packet delivery across interconnected networks

• Path determination enables a router to

Evaluate the available paths to a destination

Establish the preferred handling of a packet

Uses the IP routing table to do this

• After the router determines which path to use

it takes the packet from one interface and forwards it to another interface (switching)

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Determine L2 and L3 Address

• L3 address

Used to route the packet from the source network to the destination network

• L2 address

Delivery within a network

To get packets from one router to the next

• The IP source and destination headers do not change

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Determining Route Administrative Distance

• Administrative distance of the route

Used by router to decide the best path to the destination

A number that measures the trustworthiness

Lower administrative distance is more trustworthy the source

• Default administrative distance

RIP - 120

IGRP -100

OSPF – 110

• A route is not installed in the routing table if the administrative distance from another source is lower

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Determining Route Metrics

• metrics

Determine the best route to a destination

Measures the desirability of a route

The smaller the metric value the better the path

• RIP – hop count

• IGRP – bandwidth, delay, (reliability, load)

• Static values

bandwidth and delay don’t change

• Dynamic values

Reliability and load change between interfaces

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Determining the Route Next Hop

When a router receives an incoming packetRouter checks the destination address Router attempts to associate this address with a next hop

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Determine the last Routing Update

• Show ip route

• Show ip route <network address>

• Show ip protocols

• Show ip rip database

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Observing Multiple Paths to Destinations

• Some routing protocols support multiple paths to the same destination

• Multi-path algorithms

permit traffic over multiple lines

provide better throughput

more reliable

• IGRP uses load balancing

• Show ip route

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Network Testing

Start testing network at Layer 1

Work tests up to layer 7

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Layer 1 Errors• Broken cables

• Disconnected cables

• Cables connected to the wrong ports

• Intermittent cable connection

• Wrong cables used for the task at hand

rollovers, crossover and straight-through cables

• Transceiver problems

• DCE cable problems

• DTE cable problems

• Devices turned off

Indicator lights are a useful tool for troubleshooting

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Layer 2 Errors

• 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

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Layer 3 Errors

• Routing protocol not enabled

• Wrong routing protocol enabled

• Incorrect IP addresses

• Incorrect subnet masks

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Layer 3 troubleshooting using Ping

• Used to test network connectivity (Layer 3)

• ping sends a packet to the destination host and then waits for a reply packet from that host (echo protocol)

• Results help evaluate

path-to-host reliability

delays over the path

whether the host can be reached or is functioning

• ping uses Internet Control Message Protocol (ICMP)

Verifies hardware connection

Verifies logical address of the network layer

5 datagrams sent

Success = !

Timout = .

• Extended Ping also

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Layer 7 troubleshooting using Telnet

• telnet normally used to

connect remote devices

gather information

run programs

• Telnet application provides

A virtual terminal for connection to routers running TCP/IP

Useful to verify that a connection can be made using Telnet

• A successful Telnet connection indicates that the upper-layer application and the services of lower layers are functioning properly

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Troubleshooting L1& L1 using show interface <specific interface>

• Show interfaces displays the status of

physical hardware (layer 1)

logical software (layer 2)

Part 1 is configurationL2 problems

Part 2 is hardwareLine statusL1 problems

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• A high number of interface resets means too many keepalives have been missed. Caused by:

Bad line causing carrier transitions

Possible hardware problem at the CSU, DSU, or switch

• Use the clear counters command to reset the counters to zero after an interface problem has been corrected

• Starting from zero gives a better picture of the current status of the network

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• Keepalives

Messages sent by one network device to inform another network device that the virtual circuit between the two is still active

• If the interface misses three consecutive keepalives, the line protocol is marked as down

• If the line is down

Layer 1 problem

Cabling, connectors, equipment powered off or malfunctioning

Administratively down - manually disabled in the configuration

• Interface is up and line protocol is down

Layer 2 problem exists

No keepalives, No clock rate, Mismatch in encapsulation type

• When the line is down, the protocol is always down

no useable media for the Layer 2 protocol

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Troubleshooting using Cisco Discovery Protocol

• CDP advertises device information to its direct neighbors

MAC address, IP addresses, outgoing interfaces

• show cdp neighbors

Displays information about directly connected neighbors

• show cdp neighbors detail

displays active interfaces, port ID, device, IOS version

• For security reasons CDP should be

Configured only on links between Cisco devices

disabled on user ports or links not locally managed

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Troubleshooting using traceroute

• traceroute command is used to

Discover the routes that packet take on way to destination

Test network layer (Layer 3) on a hop-by-hop basis

provide performance benchmarks

• Traceroute output

A list of hops that were successfully reached

Every router that the datagram passes through.

indicate the specific hop at which the failure is occurring

* indicates the packet failed

round trip time (RTT) - relative performance of links

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Troubleshooting using show ip protocols

• displays values about IP routing protocol information on the entire router

which protocols are configured

which networks are being advertised

which interfaces are sending updates

timers, filters, route summarization, route redistribution

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Troubleshooting using show ip route

• Displays the contents of the IP routing table

entries for all known networks and subnetworks

how that information was learned

• used to verify that the router has a route to that network

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Troubleshooting usingshow controllers serial

• determine the type of cable connected without inspecting the cables

• Useful for finding a serial interface with

No cable

The wrong type of cable

Defective cable

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Debug command

• Produces high processor overhead

• Should be used isolate problems

not monitor normal network operation.

• By default, the router sends the debug output and system messages to the console.

terminal monitor command cam redirect this to a remote terminal

• Turn off diagnostics

no debug all or undebug all

• The time when debug events occurred

timestamps

• GAD(config)#service timestamps debug uptime