Loopback interface IP address?

cdoidge · ‎08-16-2005

Hi,

very basic question. All documentation says that loopback interface should be set up on cisco routers since it never goes down, useful for remote configuration ,etc. My question is simple - how to select IP address for it ?? Lets say that I have ISDN router with Dialer interface unnumbered, using loopback's IP. The other interface is connected to Ethernet LAN. How should I pick up IP for loopback interface, is there any guidelines or something ?

Many thanks.

Alex

amit-singh · ‎08-16-2005

Alex,

There are 2 advantages of using Loopback addresses.

The one you have already mentioned i.e loopback interface should be set up on cisco routers since it never goes down, useful for remote configuration ,etc

The another advantage is the it is administrator defined choice i.e it depends on the adnin and it can be assigned any IP as the admin wants.

So there is no specific rules that you follow, you can give any Ip as far as its is valid.

regards,

-amit singh

tal · ‎08-16-2005

It is important that the loopback addresses you pick are unique, since some routing protocols default to using the loopback IP as an id number for that router.

Just like a large organization plans their IP address allocations, some also plan their Cisco loopback address allocations.

I know one company that had a scheme like aa.bb.cc.dd where cc was the building number (they had less than 255!) and dd was a unique number for that building.