Re: Discard route Null0

Hello Jack,

Regarding the summary route produced by the ABR, I guess it is also produced
by ASBR also when doing summarization. The Null Route is produced on the
ABR, and is doing the following:
1. When the ABR normally is generating a summary address for something thus
it means it should have the subcomponents of the summary address being
advertised. In normal cases when the packet arrives to the ABR to reach one
of those subnets it should match the subcomponent (subnets) included in the
summary route and not the summary route itself. In your example it would
match 1.1.1.0/24 and not 1.1.0.0/22.
2. When the ABR for instance loses the subnet (1.1.1.0/24) the summary route
is still advertised, however when a packet needs to reach that subnet which
is not there currently, the ABR will send the route to Null 0 (discard the
route).

You can remove the null route from the routing table by using:
discard-route (internal/external). Internal is for routes summarized by the
ABR, External is for routes summarized by the ASBR (E1/E2) routes.

Hope this helps.

Best Regards,

On Sun, Aug 1, 2010 at 8:48 AM, Jack Router <pan.router_at_gmail.com> wrote:

> Hello Experts,
>
> When summarising routes in OSPF a discard route to Null0 is automatically
> created on ABR where summarisation is performed. For example:
>
> O 1.1.0.0/22 is a summary, 00:00:02, Null0
> O 1.1.1.0/24 [110/65] via 10.1.12.1, 00:00:02, Serial1/0.21
> O 1.1.2.0/24 [110/65] via 10.1.12.1, 00:00:02, Serial1/0.21
> O 1.1.3.0/24 [110/65] via 10.1.12.1, 00:00:02, Serial1/0.21
>
> I am reading in my workbook that a Null0 route purpose is to stop
> forwarding
> loops. Can anyone please explain how this loop prevention works ? Why then
> there is a command to remove the Null0 route ? In the lab I do what I am
> told but in the real life is there a reason to remove a Null0 route ?
>
> Thanks,
>
>
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-- 
KJ
Blogs and organic groups at http://www.ccie.net