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3422 Views 6 Replies Latest reply: Oct 8, 2011 9:28 PM by Naren RSS

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sourcing from loopback - why?

Aug 20, 2011 2:37 AM

alef 82 posts since
Mar 19, 2011

Maybe a bit of a thick question, and i think i sort of understand it, but why do we always ping (or i always see) in labs to ping from the loopback address. I would like to see a good and clear explanation for it but so far i haven't been able to find it.


i think it is because we do not advertise directly connected routes, and because all the routers have

a loopback address they are in the same network so they know how to get back. If we don't ping from the loopback it might source it from a address that the foreign network doesn't know how to get back to because it isn't advertised. so if we use the loopback as a source, if there is a any connection to that remote router or network available at all, it will take it back.


I don't know if this is correct or not, and i'm not even sure i understand it myself completely.


Thanks to anyone who can point me in the right direction !



  • Paul H 309 posts since
    Dec 1, 2010
    Currently Being Moderated
    1. Aug 20, 2011 5:51 AM (in response to alef)
    Re: sourcing from loopback - why?

    Hi Alef


    In lab environments loopback interfaces are can be created to emulate local networks connected behind the router when its not practical to have a bunch of switches and PCs connected to the router. In this case sourcing the ping from the loopback interface is effectively the same as running ping from a PC that would connected to the router.


    Of couse there are lots of other uses for loopback interfaces on a router and the above is only one example.


    Kind Regards



  • Paul Stewart  -  CCIE Security, CCSI 6,952 posts since
    Jul 18, 2008
    Currently Being Moderated
    2. Aug 20, 2011 6:24 AM (in response to alef)
    Re: sourcing from loopback - why?

    Testing from a loopback or any specific IP or interface, tests the route back to that specific IP (the path that the echo-reply will take) as well is the route forward.  If you don't specify a source, IOS selects the egress interface based on the route table.  The IP of the egress is used as the source of the echo.

  • Martin 13,067 posts since
    Jan 16, 2009
    Currently Being Moderated
    3. Aug 20, 2011 11:17 AM (in response to alef)
    Re: sourcing from loopback - why?

    you can build BGP peers based on loopbacks;

  • Naren 221 posts since
    Feb 3, 2009
    Currently Being Moderated
    4. Aug 22, 2011 11:33 AM (in response to alef)
    Re: sourcing from loopback - why?

    Loopbacks are the most reliable IP address than any other IPs on a router.


    The reason behind is, when your intention is to check wheter a rotuer is live and reachable via any path, it makes more sence to ping the loopback as it can go down only when the router is turned off or completely isolated.


    On the other hand, if If you use an interface's IP for testing availability of a rotuer, then you have a chance where the interface you are trying to reach is down, but the rotuer is still up  (reachable via an alternate link/interface) and you can end up deciding that the router itself is down or not reachable.


    Best Regards,



  • Naren 221 posts since
    Feb 3, 2009
    Currently Being Moderated
    6. Oct 8, 2011 9:28 PM (in response to alef)
    Re: sourcing from loopback - why?

    You are right !


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