Download CCNA2 3.1-09 Basic Router Troubleshooting

Module 9
Basic Router
Troubleshooting
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The Routing Table and show ip route
Command
• Routers build routing tables to:
– list available routes for forwarding data
– select the best path to a destination
• show ip route displays routing table information about
all known networks and subnetworks
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Example Routing Table
•When RTA receives a packet destined for 192.168.4.46,
it looks for the prefix 192.168.4.0/24 in its table.
•RTA then forwards the packet out an interface
(Ethernet0) based on the routing table entry.
•If RTA receives a packet destined for 10.3.21.5, it sends
that packet out Serial 0
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Example Routing Table
•4 routes – directly connected
•The router will drop any packet destined for a network
not listed in the routing table
•So what do you do with those packets that this router
doesn’t have a route to?
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Adding Routes
• Static routing
– Manually entered by the administrator:
• more secure
• low processor overhead
• have precise control of path selection
• Dynamic routing
– router learns route from other routers, dynamically
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Gateway of Last Resort
(Default Static Route)
Two ways to statically configure default
routes:
• From global configuration mode
– ip route 0.0.0.0 0.0.0.0 [next-hop
interface|exit-interface]
• From router configuration mode
– ip default network [network number]
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ip default-network
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ip route
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Routing Processes
• Remember that the IP source and destination do not
change in the packet, but the MAC addresses do
change.
• The packet is forwarded based on the MAC address of
the next hop.
• Layer 3 address is always used in routing, but the
MAC address has to change because the data is
framed as it moves out on the data link layer.
Source to destination --- we are always talking about
network addressing
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Route Metrics
• Routing protocols use metrics to determine the best
route to a destination.
• The metric is a value that measures the desirability of
a route.
• The smaller the metric number, the better the path.
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M
E
T
R
I
C
S
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Route Metrics
• The more factors that make up a metric, the greater the
flexibility to tailor network operations to meet specific
needs.
• By default, IGRP uses the static factors bandwidth and
delay to calculate a metric value (so bandwidth and delay
can actually be entered by the administrator).
• IGRP may also be configured to include the dynamic
factors, load and reliability, in the metric calculation.
• Remember, IGRP uses the metric
• Metric = bandwidth + delay
Be sure to do the Interactive Media Activity in 9.1.6
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Troubleshooting
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Testing by OSI Layers
• Layer 1 – check obvious first (lights on a NIC)
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Devices turned off
Disconnected cables
Broken cables
Cables connected to the wrong ports
Intermittent cable connection
Wrong cables used for the task at hand (must use
rollovers, crossover cables, and straight-through
cables correctly)
– Transceiver problems
– DCE cable problems
– DTE cable problems
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Testing by OSI Layers
• Layer 2 errors:
– Improperly configured serial interfaces
– Improperly configured Ethernet interfaces
– Improper encapsulation set (HDLC is default for
serial interfaces)
– Improper clock rate settings on serial interfaces
– Network interface card (NIC) problems
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Testing by OSI Layers
• Layer 3 errors:
– Routing protocol not enabled
– Wrong routing protocol enabled
– Incorrect IP addresses
– Incorrect subnet masks
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ping Utility (Layer 3 testing)
Maybe the host isn’t available
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Telnet application (Layer 7 testing)
• Actually verifies all seven layers of the OSI
• If telnet is not working, then that means that there are
upper-layer application layer problems.
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show interface serial
•Used to discover Layer 1 & Layer 2 problems with the
router
•When the line is down, the protocol is always down,
because there is no useable media for the Layer 2 protocol.
•If the interface is up and the line protocol is down, a Layer 2
problem exists (no keepalives, no clock rate, encapsulations
different)
•You would never see serial 0/0 is down, line protocol is up
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show interface serial
Interpreting the Show Interface Serial Command
One of the most important elements of the show
interface serial command output is the display of the
line and data link protocol status.
The line status is triggered by a Carrier Detect signal, and
refers to the physical layer status. However, the line
protocol, triggered by keepalive frames, refers to the data
link framing.
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show interfaces serial
Know all
of these!
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show ip protocols
The show ip protocols command shows the following:
• Sources of routing updates
• The networks being advertised
• Update timer values
– Example: If a network is not being included in the
routing updates and should be, an administrator could
issue this command to see which networks are being
advertised
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debug
• Be Careful!!!
• Debug will give you live, changing information.
• Therefore it will use processor time that may disrupt
normal router operation
• Especially should not use “debug all”, but be more
specific, like
– debug ip rip (only live info on rip routing ip)
• To turn off debug, use undebug all or u all
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Module 9
Basic Router
Troubleshooting
END
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