From: Divin Mathew John (divinjohn@gmail.com)
Date: Sun Mar 29 2009 - 18:56:53 ART
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
>>
>> _______________________________________________________________________
>> Subscription information may be found at:
>> http://www.groupstudy.com/list/CCIELab.html
Blogs and organic groups at http://www.ccie.net
This archive was generated by hypermail 2.1.4 : Mon Apr 06 2009 - 06:44:08 ART