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28851 Views 5 Replies Latest reply: Aug 23, 2011 6:49 AM by W.S.H. RSS

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What is a Null0 interface?

Jan 26, 2009 6:14 PM

Fahad 38 posts since
Jan 25, 2009

 

What is the purpose of Null0 interface in a summary adress? What specific role does it have in EIGRP?

 

 

 

 

 

Regards,

 

 

 

 

 

Fahad.

 

 

  • Scott Morris - CCDE/4xCCIE/2xJNCIE 8,396 posts since
    Oct 7, 2008
    Currently Being Moderated
    1. Jan 26, 2009 8:44 PM (in response to Fahad)
    Re: What is a Null0 interface?

     

    The Null0 "interface" is a virtual bit-bucket. Meaning it's a trash can to route things to. There's automatically a Null0 route (called a Discard Route) created when you do a summary-address or area-range or things like that with routing protocols.

     

     

    The idea behind this is that you are creating a summary address manually to tell other routers about. If you are therefore expecting to receive packets for anything within that summary range, what will your router do about it? if it has more specific routes, packets will get routed perfectly fine.

     

     

    Anything not matching more specific routes would get sent to the discard route and therefore destroyed in a flash of Null0 mayhem. Without the discard route, your router would be either trying to look things up or possibly sending to a default route (0.0.0.0/0) it has in its table. This in turn, may cause a routing loop where the router with the 0/0 route contains the summary you manually advertised and so packets bounce needlessly until the TTL expires!

     

     

    It's a protection mechanism.

     

     

    You may also use Null0 as a destination in static routes if you would like particular packets to die a horrible death as well!

     

     

     

     

    HTH,

     

     

    Scott

     

     

     

     

     

  • Conwyn 7,914 posts since
    Sep 10, 2008
    Currently Being Moderated
    3. Jan 27, 2009 4:17 AM (in response to Fahad)
    Re: What is a Null0 interface?
  • Scott Morris - CCDE/4xCCIE/2xJNCIE 8,396 posts since
    Oct 7, 2008
    Currently Being Moderated
    4. Jan 27, 2009 2:41 PM (in response to Fahad)
    Re: What is a Null0 interface?

     

    As long as one "candidate route" (route within the summary) exists, the summary will still be advertised. As for packets destined for a route that went down, well, they'd go to Null0 as that summary would likely be the "next best" path to the original destination that's no longer there.

     

     

    HTH,

     

     

    Scott

     

     

     

     

     

  • W.S.H. 1 posts since
    May 13, 2011
    Currently Being Moderated
    5. Aug 23, 2011 6:49 AM (in response to Fahad)
    Re: What is a Null0 interface?

    I'm having Cisco ASA 5540 , i want to monitor traffic of that ASA using netflowi'm using Solarwinds Netflow Analyzer, when i configure ASA, software is detecting all interfaces of ASA, but Solawinds showing that all traffic is going through Null 0 interface

     

     

    what is the reason for this ?

    how can i monor traffic by interface level?

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