why do we use loopback address

[ngopinath]

hi,

Can anyone pls explain why do we use loopback address & what do we mean by ip unnumbered.

Regds,

Gopi

[dgahm]

Loopback interfaces are always up, so will be reachable if any interface on the router is connected to the rest of the network.

IP unnumbered was commonly used in the old days of classful routing protocols to conserve address space. Lets say you used RIP v1 with a class B network of 172.16.0.0 and 8 bit subnetting. All interfaces, including serials, would have the same mask of 255.255.255.0. Using IP unnumbered you could share an address with another interface, avoiding wasting 252 addresses by dedicating a subnet to a serial link.

IP unnumbered is rarely used today with the widespread adoption of VLSM protocols like EIGRP and OSPF. You can use a 30 bit mask subnet, and avoid the confusion, and troubleshooting shortcomings that go with IP unnumbered.

[ahmadbilal]

hi,

Loopback address is a logical interface which is always up.So the major use is with routing protocols which require Router ID (loopback interface becomes RID).The advantage is loopback interface never goes down so RID is constant.

IP unnumbered is used to utilize a ip address you have for another interface.Althuogh not recommended for troubleshooting purposes, but if you are low on Ip addresses you can use it.

Hope this helps

[ngopinath]

hi ahmadbilal,

Thanks a lot , can u pls explain more clearly for this particular conf

interface Loopback10

ip address 172.*.*.* 255.255.255.252

interface Serial1/4

backup delay 50 60

backup interface Dialer1

ip unnumbered Loopback10

encapsulation ppp

crypto map pix

interface BRI5/0

ip unnumbered Loopback10

encapsulation ppp

dialer pool-member 1

isdn switch-type basic-net3

ppp authentication chap

interface Dialer1

ip unnumbered Loopback10

encapsulation ppp

no ip split-horizon

dialer pool 1

dialer string ******

dialer-group 1

ppp authentication chap

crypto map pix

Regds,

Gopi

[Richard Burts]

Gopi

There are a couple of things about ip unnumbered which the previous explanations did not cover explicitly and I think it is helpful to understand them.

- ip unnumbered can be used only on point to point interfaces.

- when you configure a point to point interface with ip unnumbered you point to another interface on the router which has an ip address. The unnumbered interface will borrow the ip address from the referenced interface.

So what the configuration that you ask about is doing:

- it creates a loopback interface and assignes an ip address to it of 172.*.*.* 255.255.255.252.

- interface serial 1/4 is a point to point connection and is told to use the 172.*.*.* as its ip address. So whatever is connected over the serial will see this router as having ip address 172.*.*.*

- interface bri5/0 is also told to use the 172.*.*.* as its ip address.

- interface dialer 1 is also told to use the 172.*.*.* as its ip address.

So you basically have 4 interfaces on the router all using the same ip address. This is not a problem of duplicate addresses as it might seem at first

HTH

Rick

[ngopinath]

hi Rick,

Thanks a lot for ur help, i am now through with this.

Regds,

Gopi

[farancci]

To pariticpate in public domain do we need to use public ip address on loopback interface?