Transcript CCNA2 3.0-09 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 0/0.
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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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Add routes needed by:
• Static routing
– administrator types them in, so they
are:
• 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)
2 ways to statically configure default routes:
– ip default network [network number]
– ip route 0.0.0.0 0.0.0.0 [next-hop interface or
exit-interface]
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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’re 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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RIP uses Hop
count ONLY
M
E
T
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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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Broken cables
Disconnected 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
Devices turned off
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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 clockrate 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 interfaces serial 9.3.1 (error in graphic 1)
Interpreting the Show Interfaces Serial Command
One of the most important elements of the show
interfaces 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.
Know all of these!
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Show interfaces
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Used to discover Layer 1 & Layer 2 problems w/ 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, encap different)
You would never see serial1 is down, line protocol is up
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Show ip protocols
• Shows sources of routing updates
• The networks being advertised
• Update timer values
– Ex. 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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