CCNA2
Module 9
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One of the primary functions of a router is to
determine the best path to a given destination.
A router learns paths, also called routes, from an
administrator's configuration or from other routers by
way of routing protocols.
Routers store this routing information in routing
tables using on-board dynamic random access
memory (DRAM).
A routing table contains a list of the best available
routes
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It is not feasible, or even desirable, for a
router to maintain routes to every possible
destination
Instead, routers keep a default route, or a
gateway of last resort. Default routes are
used when the router is unable to match a
destination network with a more specific
entry in the routing table
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Default routing begins with the administrator
Before routers can dynamically exchange
information, an administrator must configure
at least one router with a default route
ip default-network
or
ip route 0.0.0.0 0.0.0.0
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After configuring a default route or default
network, the command show ip route will
show the following:
Gateway of last resort is 172.16.1.2 to
network 0.0.0.0
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at each interface, as the packet moves
across the network, the routing table is
examined and the router determines the next
hop.
The packet is then forwarded using the MAC
address of that next hop.
The IP source and destination headers do
not change, at any time.
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The Layer 3 address is used to route the packet from
the source network to the destination network.
The source and destination IP addresses remain the
same. The MAC address changes at each hop or
router
A data-link layer address is necessary because
delivery within the network is determined by the
address in the Layer 2 frame header, not the Layer 3
packet header.
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The administrative distance of the route is
the key information that the router uses in
deciding which is the best path to a particular
destination.
The administrative distance is a number that
measures the trustworthiness of the source
of the route information. The lower the
administrative distance, the more trustworthy
the source
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Different routing protocols have different
default administrative distances
A route is not installed in the routing table if
the administrative distance from another
source is lower
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Routing protocols use metrics to determine
the best route to a destination. The metric is
a value that measures the desirability of a
route.
Some routing protocols use only one factor
to calculate a metric
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IGRP calculates the metric by adding the
weighted values of different characteristics of
the link to the network in question
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In the following example the values
bandwidth, bandwidth divided by load, and
delay, are weighted with the constants K1,
K2, and K3.
Metric=K1 *Bandwidth + (K2 *
Bandwidth)/256-load) + K3 * Delay
The default constant values are K1=K3=1
and K2=K4=K5=0 so:
Metric=Bandwidth + Delay
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Some routing protocols support multiple
paths to the same destination. Unlike single
path algorithms, these multi-path algorithms
permit traffic over multiple lines, provide
better throughput, and are more reliable
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The commands telnet and ping are two
important commands that are used for
testing a network.
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Layer 1 errors can include:
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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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Layer 2 errors can include:
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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 can include:
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Routing protocol not enabled
Wrong routing protocol enabled
Incorrect IP addresses
Incorrect subnet masks
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The ping command is used at Layer 3 to test
connectivity.
At Layer 7 the telnet command may be used
to verify the application layer software
between source and destination stations.
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Ping
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The ping command can be invoked from
both user EXEC mode and privileged EXEC
mode.
The ping command can be used to confirm
basic network connectivity
To use extended ping, type ping at the
command line, then press the Enter key
without entering an IP address
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Telnet
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The telnet utility is a virtual terminal protocol
that is part of the TCP/IP protocol suite. It
allows verification of the application layer
software between source and destination
stations.
This is the most complete test mechanism
available. The telnet utility is normally used to
connect remote devices, to gather
information, and to run programs.
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Troubleshooting Router
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The Cisco IOS contains a rich set of
commands for troubleshooting. Among the
more widely used are the show commands
show interfaces
show interfaces FastEthernet
show interfaces serial0/0.
show version
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This is determined by whether keepalives are
successfully received.
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If the show interfaces output shows the last
clearing of the counters as never, use the
show version command to find out how long
the router has been functional.
Use the clear counters command to reset
the counters to zero
These counters should always be cleared
after an interface problem has been
corrected
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The show interfaces command is perhaps
the single most important tool to discover
Layer 1 and Layer 2 problems with the
router.
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The first parameter (line) refers to the
physical layer
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The second parameter (protocol) indicates
whether the IOS processes that control the
line protocol consider the interface usable
This is determined by whether keepalives are
successfully received.
Keepalives are defined as messages sent by
one network device to inform another
network device that the virtual circuit
between the two is still active
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If the interface is up and the line protocol is
down, a Layer 2 problem exists. Among the
possible causes are:
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No keepalives
No clock rate
Mismatch in encapsulation type
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Troubleshooting using show cdp
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The output from the show cdp neighbors
command displays information about directly
connected neighbors
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Troubleshooting using traceroute
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The traceroute command is used to
discover the routes that packets take when
traveling to their destination.
Traceroute can also be used to help test the
network layer (Layer 3) on a hop-by-hop
basis and provide performance benchmarks.
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Troubleshooting using show ip route
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The show ip route command is perhaps the
single most important command for
troubleshooting routing issues
This command displays the contents of the
IP routing table.
The output from the show ip route
command shows the entries for all known
networks and subnetworks, and how that
information was learned
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Troubleshooting using show ip
protocols
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The show ip protocols command displays
values about IP routing protocol information
on the entire router.
This command can be used to confirm which
protocols are configured, which networks are
being advertised, which interfaces are
sending updates, and the sources of routing
updates
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Troubleshooting using show
controllers serial
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By examining the show controllers serial
command output, the type of cable that the
controller detects can be determined. This is
useful for finding a serial interface with no
cable, the wrong type of cable, or a defective
cable.
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Debug
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The debug commands assist in the isolation
of protocol and configuration problems
The debug command is used to display
dynamic data and events
The dynamic output of the debug command
comes at a performance cost, producing high
processor overhead that may disrupt normal
router operation
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Verifying RIP Configuration
Dublin# show ip protocols
RIP routing is configured
Interfaces sending and
receiving RIP updates
Router is advertising the
correct networks
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Layer 1 Troubleshooting
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Check power.
Check the NIC or interface’s ‘Link Light’.
Make sure interface cards are inserted correctly.
Ensure appropriate, known-good cable is correctly attached (both ends).
Verify that switch ports are in the correct collision domain (VLAN), and have
appropriate options set (spanning tree).
Check transceiver is attached and functional. Physical layer (line status)
Up: has Layer 1 connectivity
BHM# show interface s0
Down: L1 problem
Serial0 is up, line protocol is up
Administratively down: disabled
< output omitted >
Last clearing of "show interface" counters never
< output omitted >
To reset: BHM# clear counters
Received 73 broadcasts, 0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
135 packets output, 7361 bytes, 0 underruns
0 output errors, 0 collisions, 37 interface resets
0 output buffer failures, 0 output buffers swapped out
54 carrier transitions
Faulty hardware,
cable or noise
Line interruptions,
faulty hardware
Bad line,
bad hardware
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Layer 2 Troubleshooting
If the interface is up and the line protocol is down, a Layer 2 problem exists.
Indicates whether the IOS processes consider the interface usable.
If the interface misses three consecutive keepalives, line protocol is down.
When the line is down, the protocol is always down.
Data-Link layer (up or down)
BHM# show interface serial 0
Serial0 is up, line protocol is up
Are keepalives being received?
Hardware is HD64570
Internet address is 172.17.0.2/16
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely 255/255, load 1/255
Encapsulation HDLC, loopback not set, keepalive set (10 sec)
Last input 00:00:01, output 00:00:00, output hang never
Last clearing of "show interface" counters never
< output omitted >
Encapsulation must match at
both ends of connection
Router# show controllers serial 0
HD unit 0, idb = 0x1086D4, driver structure at 0x10E568
buffer size 1524 HD unit 0, V.35 DCE cable, clockrate 56000
In lab situations where the router is the
DCE the clockrate must be M.
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Layer 3 Troubleshooting
Both Ping and Traceroute are used to troubleshoot Layer 3 problems.
The ping utility is used to test basic network connectivity.
The ping command can be used with IP and most other Layer 3 protocols.
Extended ping offers a more extensive range of test options.
vista# ping SanJose1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.2, timeout is 2
seconds:
‘!’ indicate a successful echo
..!!!
Success rate is 60 percent (3/5),
‘.’ indicate the application
round-trip min/avg/max = 32/33/36 ms
timed out waiting for an echo
Traceroute is used to discover routes packets take to their destination.
Also used to test on hop-by-hop basis and provide performance benchmarks.
The last good hop can be used to indicate a problem area (failed link).
YORK# trace ROME
RTT used to approximate delay on the link
Type escape sequence to abort.
Tracing the route to ROME (172.16.33.5)
1 LONDON 192.168.1.6
16 msec 16 msec 16 msec
* packet failed
2 PARIS 192.168.1.37
16 msec
*
16 msec
3 ROME
172.16.33.5
16 msec 16 msec 4 msec
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