06-12-2006 12:16 AM - edited 03-03-2019 12:58 PM
when i say the configuration in the router When we advertise the subnets directly in BGP using network command
we are using a static route of the same full subnet pointing to NULL 0
Can any one Help me understanding this concept of NULL 0 Routing
11-21-2020 02:24 AM
I appreciate your effort to revive this old discussion. But I do not understand your question. A default route using 0.0.0.0 is familiar. But in your question what is 1.1.1.1? What do you want to do with 1.1.1.1?
11-19-2015 05:33 PM
I just noticed the question was asked 9 years ago.
02-15-2017 07:57 AM
Hello Team,
today i was reading about ospf abr summrization and saw null0 , i want to know what are advantages of Null0 and dis-advantages . if advantages of Null0 then why we run command that is
ospf discard route internal to remove Null0 entry from the network.
i have seen that because of Null0 , a looping problem also can create in our Network that is why we run ospf discard route internal
Please let me know is this correct ?
02-15-2017 08:38 AM
Hello,
Let me give you an example. Consider this senario
Internet---ASBR------ABR-------Area 0(172.16.1.0/24
172.16.2.0/24
172.16.3.0/24)
We do summarization on ABR (172.16.0.0/16)
ABR has a default route to ASBR and 172.16.0.0 to null0
And ASBR has 172.16.0.0/16 to ABR
Suppose 172.16.3.0/24 is down and there is no Null0. ASBR sends traffic for 172.16.3.0/24 to ABR(because of summary route) and ABR returns it to ASBR( because of default of route) so if there is no Null0, there would be forwarding loop.
The command you mentioned is rarely used. But suppose this scenario. Everything is the same as above, just we have 172.16.100.0/24 behind the ASBR. Suppose from different domain. Normally, it is reachable by default route on ABR (toward ASBR), but when you do summarization, it is not reachable anymore because 172.16.0.0/16 Null0 is longer prefix than default route so packets will be dropped. In this situation, we can use that command to remove Null0 from the routing table.
Hope it helps,
Masoud
02-15-2017 09:07 AM
so what is the work of Null0 ?
we have to manually this command ?
i mean ip route 172.16.0.0 255.255.0.0 null0
or
it comes during summary
area 2 range 172.16.0.0 255.255.0.0
02-15-2017 09:17 AM
It sits inside the routing table automatically after summarization. in the previous example, for instance, packets destined to 172.16.7.2 will be dropped( Null0 is a black hole), but packets destined to 172.16.2.2 will be forwarded to area 0 because 172.16.2.0/24 is longer prefix than 172.16.0.0/16 to null0.
Masoud
02-15-2017 09:24 AM
how it comes
172.16.1.0/24
172.16.2.0/24
172.16.3.0/24)
We do summarization on ABR (172.16.0.0/16)
it should be
172.16.0.0/22
08-19-2015 12:27 AM
Hi Rick, Ankur & Devang
Thanks for the detailed explanation.
Diljith
08-19-2015 03:55 AM
Diljith
You are quite welcome. I hope that this forum is helpful to you.
HTH
Rick
02-15-2017 07:57 AM
Hello Team,
today i was reading about ospf abr summrization and saw null0 , i want to know what are advantages of Null0 and dis-advantages . if advantages of Null0 then why we run command that is
ospf discard route internal to remove Null0 entry from the network.
i have seen that because of Null0 , a looping problem also can create in our Network that is why we run ospf discard route internal
Please let me know is this correct ?
11-19-2015 02:50 PM
Richard,
Thank you for the excellent explanation of the Null routes. I especially like the way you tied the previous answers together and expanded upon them.
Trent
11-20-2015 05:52 AM
Trent
Thank you for posting to the forum and telling me how much you like my explanation about using route to null 0. It is gratifying to know when my responses have helped people. These forums are excellent places to learn about networking and to share what we have learned with others. I hope you will continue to be active in the forums.
HTH
Rick
11-20-2015 07:35 AM
I also thank you Richar for your support. I was amazed when I saw that you had answered the question 9 years ago. I hope you keep up so that we can learn more from you.
Masoud,
11-20-2015 07:40 AM
Masoud
Thank you for the kind words. Yes I have been active in the forums for quite a while. And I hope to continue to be active in the forum and hope that people can learn from my responses. I am glad to see your activity in the forum with very good answers and hope that you also will continue to be active.
HTH
Rick
01-11-2020 03:03 PM
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