Re: Regarding Null 0

From: Raghav Bhargava (raghavbhargava12@gmail.com)
Date: Sun Mar 29 2009 - 19:03:17 ART

Hi divin/salah,

Can you guys please answer my questions that I requested for in my mail..I
understand in EIGRP wen we create a summary address it creates a Null 0
route but my question is if a packet comes in for a different network what
happens then..

raghav

On Sun, Mar 29, 2009 at 3:00 PM, Salah ElShekeil
<salah.elshekeil@gmail.com>wrote:

> yep in rip it won't creat route to Null you have to do it manually
>
> ip route xxxx xxxx null0
>
> On Mon, Mar 30, 2009 at 12:56 AM, Divin Mathew John <divinjohn@gmail.com>wrote:
>
>> and i do think its only in OSPF and EIGRP when u do summarize routes
>> internal or external that creates a NULL route automatically.! for rip i
>> guess will have to create a static route pointing to the Null0 with a
>> higher
>> administrative distance than any other routing protocol....to avoid
>> loops/!
>> Thanking You
>>
>> Yours Sincerely
>>
>> Divin Mathew John
>> divinjohn@gmail.com
>> divin@dide3d.com
>> +91 9945430983
>> +91 9846697191
>> +974 5008916
>> PGP PUBLIC KEY BLOCK @ http://www.dide3d.com/divin_Public_PGP_key.txt
>> Sent from: Bangalore KA India.
>>
>> On Mon, Mar 30, 2009 at 3:25 AM, Divin Mathew John <divinjohn@gmail.com
>> >wrote:
>>
>> > 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
>> > +974 5008916
>> > PGP PUBLIC KEY BLOCK @ http://www.dide3d.com/divin_Public_PGP_key.txt
>> >
>> >
>> >
>> > 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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> 

-- 
Warm Regards
Raghav

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