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I may be misunderstanding what you can do with a Router's loopback interface. I am trying to ping it from a PC on the same network.

I am using Packet Tracer to simulate this, a Cisco 1841 and a PC from within the program.

IPs

Cisco 1841:

interface Loopback0
 ip address 192.168.254.1 255.255.255.0
!
interface FastEthernet0/0
 no ip address
 duplex auto
 speed auto
!
interface FastEthernet0/1
 no ip address
 duplex auto
 speed auto

PC:

IP -- 192.168.254.5 255.255.255.0

enter image description here

Diagram from Packet Tracer

enter image description here

Not sure if this is possible, or maybe I'm really not understanding the loopback interface, but I am trying to dive into OSPF and want to understand other ways to use the Loopback Interfaces on Routers.

If I am completely wrong, just let me know :)

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1 Answer 1

up vote 2 down vote accepted

As with the loopback interface on your PC, loopback interfaces on Cisco devices are local to those devices only. In order to reach the loopback IP, you must have some route to it via the Cisco device in question.

In your example, you would need to give the router an address on Fa0/1 and put the PC in the same subnet (not the same subnet as the loopback interface). The PC will then need a route to the loopback IP via the 1841. This can be a default route, or a more specific route, just so long as the PC knows that it had to get to the 1841's loopback IP via the 1841's Fa0/1 IP.

For example:

! Change the IP here and use a /32 netmask
interface Loopback0
 ip address 192.168.255.1 255.255.255.255

! give the old Loop0 IP to Fa0/1
interface FastEthernet0/1
 ip address 192.168.254.1 255.255.255.0

Your PC already has an address inside 192.168.254.0/24, so all you need to do is configure its default gateway as 192.168.254.1. The PC should now be able to ping the 1841's loopback IP, 192.168.255.1.

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Thank you James. Also, make sure the PC has the default gateway set to the Fa0/0 interface's IP address or the PC won't be able to hit the x.x.255.1 loopback0 address on the router. –  Jim Feb 28 '13 at 18:32
    
Unless the Cisco router has proxy-arp enabled, which most do by default. Although I don't know if Packet Tracer correctly simulates this behavior. –  cpt_fink Mar 6 '13 at 4:27

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