loopback interface in router

Unanswered Question
Mar 27th, 2008
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R1:


>enable

#conf term

(config)# int ser 0

(config-in)#ip address 192.168.1.1 255.255.255.0

(config-in)#no shutdown

(Config-in)#end

#

R2:

>enable

(config)#int ser 0

(config-if)#ip address 192.158.1.2 255.255.255.0

(config-if)#no shutdown

(config-if)#exit

(config)#interface loopback 1

(config-if)#ip address 192.168.1.10 255.255.255.0

(config-if)#end

#

R1:

>enable

#ping 192.168.1.10



I have configured my two router with the serial interface and one loopback interface in R2 router as above commands



My query is why we are using loopback interface there in R2 ? as per my knowledge i know that the loopback nterface is a logical interface which will be UP unless the router that is in my case R2 powered off.


But if the physcial link between the router that is R1 and R2 goes down, whether we can able to ping and telnet the logical interface in R2 from R1


please suggest on the above issue and let me know the much more details about loopback interface.


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Overall Rating: 4 (2 ratings)
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Richard Burts Thu, 03/27/2008 - 08:23
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vinoth


In general we configure a loopback interface because there is some use that we want to make of it. Perhaps that use is to be the RouterID for OSPF, or perhaps that use is to be the source interface for certain types of traffic (perhaps syslog, or NTP, or TACACS/Radius, or something else), or perhaps that use is that if the router has more than one way to get to it we can access the router by loopback without being concerned about which physical path we use.


There are several problems with what you have configured:

- the IP addresses on the serial interface do not match (check the second octet) and so you will not have IP connectivity over the serial link.

- you should not configure a loopback with /24 mask that is in the same subnet as some other router physical interface (with /24 mask).


HTH


Rick

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