From: Divin Mathew John (divinjohn@gmail.com)
Date: Sun Mar 29 2009 - 18:55:09 ART
> The Null interface is typically used for preventing routing loops. Enhanced
> Interior Gateway Routing Protocol (EIGRP), for instance, always creates a
> route to the Null0 interface when it summarizes a group of routes. Whenever
> a routing protocol summarizes, this means that the router might receive
> traffic for any IP address within that summary. Because not all IP addresses
> are always in use, there is a risk of looping packets in case default routes
> are used on the router which receives the traffic for the summary.
http://www.cisco.com/en/US/tech/tk364/technologies_tech_note09186a00801c9a6e.shtml
refer that doc. u will know why null0 is created
and
http://www.cisco.com/en/US/tech/tk364/technologies_tech_note09186a00801c9a6e.shtml
However, in IOS levels 12.1(6) and higher, this discard route is generated
by default, and you don't need to create it. If you want to disable creation
of the discard route, use the *no discard-route* command:
> Router1(config)#*router ospf 55*
> Router1(config-router)#*no discard-route internal*
>
> Router1(config-router)#*no discard-route external*
>
> With the *internal* keyword, this command prevents the router from
automatically generating discard routes for internal summary routes.
Similarly, the *external *keyword is for external routes. However, we urge
caution with this command because the absence of a discard route can cause
loops.
Thanking You
Yours Sincerely
Divin Mathew John
divinjohn@gmail.com
divin@dide3d.com
+91 9945430983
+91 9846697191
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On Mon, Mar 30, 2009 at 2:27 AM, Raghav Bhargava <raghavbhargava12@gmail.com
> wrote:
> Hi Experts,
>
> Have some doubts related to Null 0.
>
> In EIGRP,
>
> For example if we have 4 networks:-
>
> 1.1.0.1/24 , 1.1.1.1/24, 1.1.2.1/24, 1.1.3.1/24
>
> Now lets say I summarise them all under 1 default route by the command ip
> summary -address eigrp 10 0.0.0.0 0.0.0.0
>
> when I do this it creates a Null 0 route in the Routing table. So If I
> would
> have not summarise these networks as a default route and would have done it
> like this ip summary-address eigrp 10 1.1.0.0 0.0.3.255 .Then will the Null
> 0 route be created.???? I don know..
>
> As per my understanding of Null 0 , it helps in preventing forwarding Loops
> . So does it mean that if i want to reach a network which is outside of
> these 4 networks for eg - 4.4.4.4/24. The Null 0 will drop the route for
> this network. Please correct me if I am wrong.
>
> Secondly like in OSPF also when we do summarisation of internal /external
> routes a null 0 route is created. I know it is used for preventing
> forwarding loops but will it do the same thing . I mean if a route for a
> different network comes in will it drop it.
>
> Also in OSPF we can remove this Null 0 via the command---> no discard-route
> internal/external. Why do we do that if Null 0 helps in preventing the
> loops.
>
> Last but not the least is there a command to do the same in other protocols
> like RIP, EIGRP, BGP.
>
> just curiosu to know ....
>
>
> --
> Warm Regards
> Raghav
>
>
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