Regarding Null 0

From: Raghav Bhargava (raghavbhargava12@gmail.com)
Date: Sun Mar 29 2009 - 17:57:11 ART

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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