From: Divin Mathew John (divinjohn@gmail.com)
Date: Sun Mar 29 2009 - 19:06:55 ART
See the Null0 is for preventing loops as in..
u have this scenario
R1 <-----> r2 <====> ISP
R1 adverstises 4.0.0.0 summary address....and R1 has default route to R2.
now if a packet comes from the ISP to R2 for say 4.4.4.2 and it doesnt exist
then R1 will forward the packet out to R2 due to default route.! and R2 will
send it to R1 due to the summary route...and there u see is a LOOP....to
avoid thi u have a Null0 interafce!
and i guess in any proper network u wouldnt have overlapping IP addresses.!
atleast distinctive subnets.!
Thanking You
Yours Sincerely
Divin Mathew John
divinjohn@gmail.com
divin@dide3d.com
+91 9945430983
+91 9846697191
+974 5008916
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Sent from: Bangalore KA India.
On Mon, Mar 30, 2009 at 3:33 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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