From: Salah ElShekeil (salah.elshekeil@gmail.com)
Date: Sun Mar 29 2009 - 19:00:36 ART
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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