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Community Member

LoopBack Addressing

I need to know how to configure a router to use a Loopback address as the Source address of a packet when it goes out another interface. For example, A packet entering the Eth0/0 interface destined for a network that exists out of its Serial0/0 interface. I do not want the packet to use the Serial0/0 interface... This is possible, correct? If so , How can I implement this? - I am having trouble finding the info on CCO...

Thanks in advance.

Community Member

Re: LoopBack Addressing

First questions is why? But aside from that keep in mind that the source and destination IP addresses are of the hosts who are in session together, not the router. Unless the router is one of the host. But in normal routing the layer 3 address of the routers are not used, just the layer 2.

Community Member

Re: LoopBack Addressing

The other answer is spot on. To add to it, if you need to use the loopback as the source address for a ping on the router, use the extended ping command.

To access it, type in ping without any other parameters. when it asks for extended commands, say yes. The next prompt is for the source ip address.


Cisco Employee

Re: LoopBack Addressing

I'd like to say that when a packet comes in on interface e0 to get out on another interface, the router does not change the source ip address.

Unless you have nat configured.

You can control what source ip address the router will use for traffic generated by the router.

ping is an example but also tacacs (use the command 'ip tacacs source ...', telnet ....

Please give us more detail of what exactly you're trying to do so we can give you a more accurate anwer.

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