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Arora Aunty Teaches.....



How to configure 'IP UNNUMBERED' in Cisco Routers?

What is the advantage of a loopback interface?



interface Ethernet0
ip address


interface Serial 0
ip unnumbered Ethernet 0


Aunty why in the above commands, serial0 is not having any ip address?

For point to point lines, ip address is not that much necessary. By the above method we are saving ip addresses.


But you have written the word 'ethernet0' in the interface serial0, why aunty?

An interface cannot work without any ip address. so we have simply borrowed the ip address of ehternet0.


But if ethernet0 is down, then what will happen to serial0, it will also go down aunty?

Yes, that is one of the disadvantage with this 'ip unnumbered' concept. This can be avoided by borrowing the ip address from a LOOPBACK ip address. The loopback interfaces are VIRTUAL. so they will never go down.

Another disadvantage is : you cannot test whether this serial0 is really up or down, from a remote router.


If I put the command "show ip interface brief" what result i will get aunty?


RouterA# show ip interface brief

Ethernet0 YES manual up up
Serial0 YES manual up up


Shall i confiure 'ip unnumbered' in my ethernet interface of the router, aunty?

No, only in point to point network ,that is, only in serial links, you can configure ip unnumbered.


Can i configure 'ip unnumbered' in both the ends of a point to point link, aunty?