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
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Path determination occurs at network layer
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The network layer provides best-effort, end-toend, packet delivery across interconnected
networks
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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
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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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Do interactive media lab 9.1.4
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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
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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 packet
Router 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
Work tests up to layer 7
Start testing network at Layer 1
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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
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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 = .
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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 upperlayer 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 configuration
L2 problems
Part 2 is hardware
Line status
L1 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 using
show 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
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