What is the use of null route

[rangap26.]

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

[Richard Burts]

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?

[Masoud Pourshabanian]

I just noticed the question was asked 9 years ago.

[jain.manish94]

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 ?

[Masoud Pourshabanian]

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                                      

[jain.manish94]

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

[Masoud Pourshabanian]

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

[jain.manish94]

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

[Diljith E K]

Hi Rick, Ankur & Devang

Thanks for the detailed explanation.

Diljith

[Richard Burts]

Diljith

You are quite welcome. I hope that this forum is helpful to you.

HTH

Rick

[jain.manish94]

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 ?

[TrentGivens]

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 

[Richard Burts]

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

[Masoud Pourshabanian]

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,

[Richard Burts]

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

[Wezhildho]

Thank you for an insight Master.