From: Salah ElShekeil (salah.elshekeil@gmail.com)
Date: Sun Mar 29 2009 - 19:07:28 ART
if the router who is generating the summary have no longer match in its
routing domain then it will discard it
On Mon, Mar 30, 2009 at 1:03 AM, Raghav Bhargava <raghavbhargava12@gmail.com
> wrote:
> 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
>>> >>
>>> >>
>>> >> Blogs and organic groups at http://www.ccie.net
>>> >>
>>> >>
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>>>
>>
>
>
> --
> Warm Regards
> Raghav
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