What's the prupose of "ip route null0"?

[gwhuang5398]

We often saw configuration of putting the summary address of an internal private network in "ip route .... null0". What's the purpose of it? Why the traffic to the internal network doesn't get dropped on the router it is configured?

Would the null0 statis route show up in the "sh ip route?

Thanks

Gary

[Lefteri]

What if I wanted to drop all traffic, not just DNS from specific networks? For example the Spamhaus DROP list, wouldn't Null0 be able to do that?

[Richard Burts]

Perhaps there are aspects of your question that I am not understanding correctly. But it seems to me that when you mention the DROP list that you are thinking more of dropping traffic based on the the source address of the traffic. Using the null 0 approach would be dropping traffic based on the destination address.

Certainly if you want to drop traffic based on the destination address then the null 0 approach would work quite well.

HTH

Rick

[jain.manish94]

but here 172.17.5.5 it should be between 172.17.4.0 to 172.17.6.0 i did not get your point please can you clear more about it.

[Richard Burts]

The point is that the router does not know where 172.17.5.5 is. It certainly is not in 172.17.4.0/24 and it certainly is not in 172.17.6.0/24. You are correct that numerically 172.17.5.5 is between those two subnets. But that does not tell us anything about how to route so that we reach 172.17.5.5. If we assume that 172.17.4.0 is reached on FastEth0/0 and that 172.17.6.0 is reached on FastEth0/2, how would you suggest that we get to 172.17.5.0?

HTH

Rick

[jain.manish94]

Thanks to reply and clear it...

i have one question please can you clear it.

suppose i have two network ..

1. 192.168.10.0/30

2. 192.168.10.8/30

Now i just create one summary address for both of them that is

192.168.10.0/28 is a summary

now my question is that if in interview someone tell me that

192.168.10.0/28 is a summary .. ok

now how to know that in this summary only two network comes that are

192.168.10.0/30

192.168.10.8/30

because i created one summary address only for those two netwrok..

Please share your ans...

[Richard Burts]

I am not sure that I fully understand your question. In particular it is not clear where in the network you are looking at this summary route. And it is not clear how the summary is created and how the summary is advertised. Let us think of a simple example where router A is connected to router B which is connected to router C. Let us assume that 192.168.10.0/30 and 192.168.8.0/30 are on router A. And let us assume that you create the summary on router B using the EIGRP summary-address and advertise the summary in EIGRP to router C.

So on router A if you do show ip route you would see the two /30 subnets. On router B if you do show ip route you would see the two /30 subnets and the summary and so you could tell which subnets in the summary actually exist. But on router C you would see only the /28. And on router C there is not any way to know which subnets actually exist.

HTH

Rick

[Joseph W. Doherty]

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Posting

Yes, you could summarize those two networks using 192.168.10.0/28, but that summary actually also covers

1. 192.168.10.4/30

2. 192.168.10.12/30

As described by Rick, without knowing what else is being advertised, and where's its being advertised, we don't actually know what networks will actually arrive to the router advertising that summary.

All we know is any destination that falls within 192.168.10.0/28 might come to that router.  Remember, for example, the "undefined" address blocks might have traffic even more specific.  For example, 192.168.10.4/31 or 192.168.10.4/32.

[PK]

Hey Guys,

Even I have a question on the null0 statement.

Do I have to use the ip route... null0 if I have an exact subnet static route mentioned?

eg.

So I am advertising a subnet in bgp

network 172.16.16.0 mask 255.255.252.0

and then I have static route

ip route 172.16.16.0 255.255.252.0 172.16.19.228

ip route 172.16.16.0 255.255.252.0 Null0 200

So now here do I still have to use the null0 statement as due to the static route it will inject that subnet into the routing table, correct? And FYI.. I am receiving entire bgp table from the internet.

[Richard Burts]

One of the principles of configuring BGP is that when a BGP network statement supplies a mask then there must be a prefix in the ip routing table that is an exact match (subnet and mask) for BGP to advertise the route.

So if you have this network statement

network 172.16.16.0 mask 255.255.252.0

then you must be sure that your routing table has 172.16.16.0 255.255.252.0

are you sure that your static route of ip route 172.16.16.0 255.255.252.0 172.16.19.228 will insert that prefix into the routing table? If so then you do not need the static route using null0.

Let me suggest a few things to think about as you answer the question I just asked:

- does your router know how to reach 172.16.19.228? That address is inside the block of addresses being routed

- if the router does know how to reach 172.16.19.228 but the interface used to reach that address goes down do you want BGP to still advertise that address? Because if the interface used to reach the next hop goes down then the router will withdraw the prefix from the routing table.

HTH

Rick

[Bradley Marshall]

Fantastic explanation!

[Richard Burts]

Thank you. I am glad that you found my explanation helpful.

HTH

Rick

[Joseph W. Doherty]

One of the principles of configuring BGP is that when a BGP network statement supplies a mask then there must be a prefix in the ip routing table that is an exact match (subnet and mask) for BGP to advertise the route.

Just a footnote to Rick's comment, I recall (?) BGP can also aggregate based on what it sees in the route table.  So, you don't always need an exact 1 for 1 match, but you would need something subordinate to the aggregate to get the aggregate advertised.

[Richard Burts]

Joseph seems to be conflating two different functions in BGP. The original discussion has been about the BGP network statement including a mask. Now Joseph brings up the BGP aggregate address function.

The network statement might be used to advertise a summary (or supernet) address and could potentially use the route to null 0 to achieve the advertisement. The aggregate address can advertise a summary (or supernet) address but I do not see how the null 0 route is a factor in that function.

HTH

Rick

[Joseph W. Doherty]

Ah, I may have indeed conflated.

Only now, I noticed the difference in time stamps between the OP and then posts and PK's and later postings.

My "footnote" was based on the OP context, not just the recent discussion postings.  If you read it in that context, it might seem less conflated.

[jain.manish94]

can someone help me to configure bgp failover in Gns3 . PLEASE find attachment , i am using this diagram

1. using two loopback address 8.8.8.8 as a internet ip

2. R1 to R3 ---- EBGP

3. R2 to R4 ---- EBGP

4. on R3 and R4 ---- OSPF

5. 8.8.8.8 is showing on router R3 , but i am not able to ping 8.8.8.8 from my PC .

i did

R3#

ip prefix-list out permit 0.0.0.0/0

route-map dataout permit 10

match ip address prefix-list out

set local-preference 600

router bgp 3

nei 192.168.1.1 route-map dataout out

same on router R4

Please share , where i am wrong and what to do to ping 192.168.1.1 from my pc