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Identifying and Resolving Generic Router Problems

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DCD=down DSR=down DTR=down RTS=down CTS=down

Router_A#

This output indicates that the interface is still administratively down. You

saw the

no shut

command issued in the previous screen capture, so why is

it still in shutdown? Here is the answer. If an interface has a configuration
conflict with another interface, it will not initialize. Because the serial inter-
face was configured with a duplicate IP address, it wouldn't initialize. It
remains in its previous state, shutdown. In order to activate the serial link we
must do some more analysis.

Referring to the

show interface

results for Ethernet 0/1, you see that it

does have the same address as Serial 0/0. This problem can easily be resolved,
as long as you know which interface should have the

172.16.20.5

address.

In this scenario, we will assume that Ethernet 0/1 has the incorrect IP
address.

In essence, you have made additional observations.

Serial 0/0 is configured with IP address

172.16.20.5/30

Ethernet 0/1 is configured with IP address

172.16.20.5/24

Cannot change administrative state for Serial 0/0 because of the IP
address overlap with Ethernet 0/1.

Now, with these additional observations, new solutions must be pro-

posed. Once the decision is made as to which IP address should be assigned
to each interface, the problem should be resolved. The action plan is as fol-
lows:

Leave IP address 172.16.20.5/30 to interface Serial 0/0.

Assign IP address 172.16.30.1/24 to interface Ethernet 0/1.

Remove the administrative shutdown from interface Serial 0/0.

Here is the configuration. Following the configuration, you see the show

interface

output for each interface. This is done to verify that all the

changes that were made to the router fixed the problems that were observed.

Router_A#conf t

Enter configuration commands, one per line. End with CNTL/
Z.

Router_A(config-if)#interface ethernet 0/1

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