loopback address

[lowdown]

Can anybody give me a clear definition of what a loopback address is for a Cisco router? WHy exactly would it be used and how do you determine what the loopback address should be? Thank you

 

[eurobadger]

Hi

Cisco's glossary defines loopback address as follows: A zone that enables the server to direct traffic to itself. The host number is almost always 127.0.0.1.


Doesn't help much, though, does it?

I see it in networks where ISDN is used with ip unnumbered. In this sort of setup the BRI interface uses the (fixed) ip address of another interface on the router as its own. The snag is that the inteface whose address you borrow has to be up for the address to be usable. The loopback inteface(s)are virtual, and are always up once configured, so the address from the loopback interface is always available.

Once the loopback interface address is configured it is a valid interface on the router, and can be used as a label or name for the router when contacting it via other interfaces from remote hosts.

Extract from a config that uses loopback address in IP unnumbered:

!
!
interface Loopback0
ip address 10.123.231.132 255.255.255.255
no ip directed-broadcast
no ip route-cache
!
interface BRI0
description ISDN dial-on-demand
ip unnumbered Loopback0
no ip directed-broadcast
encapsulation ppp
no ip route-cache
dialer string 00441234567890
dialer-group 1
isdn switch-type basic-net3
custom-queue-list 1
no cdp enable
ppp authentication chap
no shut
!


Using this, when active the BRI interface will use the IP address 10.123.231.132 to communicate.

Hope that helps.

http://www.cisco.com

has other examples if you want them.

EB

 

[andf1]

A loopback interface is a virtual interface, that to the Cisco IOS software appears like any physical interface. Unlike a physical interface, it never goes down and hence has many uses. In BGP it is used to establish peering between neighbors, so that if even if the physical interface went down for a short time the session does not go down.

Andy

 

[andf1]

By the way, you can use any network address on the loopback interface. If the loopback address is included in the routing process, the route gets advertised. Choose the address carefully. Use an address that is not used elsewhere on your network. Some engineers used the loopback for managing routers and reserve a network segment for the loopbacks. For example, router 1 has the loopback 192.168.0.1/32, router 2 has the loopback 192.168.0.2/32 and so on.

Andy