Usage of null0

[mahmoodmkl]

Hi Guys,

Can anyone explain me the useage of null0 in the routing table.

Why it is used and in which situation we should use it.

Any help is appreciated.

Thanks

Mahmood

[Marwan ALshawi]

null0 used in some kinds of route blocking

which is called balck holing

for eaxmple instead of makeing ACL and block the route in ur router

u can recieve that route and make it point to null0 so it will be blocked and less CPU intenssive when u block it with ACL

for example u might want to block route going to 192.168.1.0/24

just do it like

ip route 19.168.1.0 255.255.255.0 null0

also this null0 will apear in the routing table whn u use summarization with eigrp

but when u see the summary address with null0 as next hop interface that mean this router is generating that summaty route (not blocking in this case)

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[mahmoodmkl]

Hi

Thanks for your reply.

How does it prevent from route block holing can anyone please explain me with a example.

I m missing concept here.

Thanks

Mahmood

[Marwan ALshawi]

lets take this simple example

router1----router2--10.1.1.0/24

router1 has a route to 10..1.1.0/24 point to router2 as the next hop

on router2

u make the null0 config like

ip route 10.1.1.0 255.255.255.0 null0

in this case a packet going to 10.1.1.0/24 from router1 will be send to router2 router2 will take this packet ( not drop it) then send it to the route with null0 then the route will not actually be forward to that network and then u block it but with out any deny or ACL

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[mahmoodmkl]

Hi

Thanks for your reply.

If it is used in EIGRP and EIGRP is advertising a summary route in this case how the null0 works

Thanks

Mahmood

[Marwan ALshawi]

hi Mahmood

hope evrything is well

ok with eigrp summary route the summarized route in the routing table will looks like :

D 2.1.0.0/16 is a summary, 00:00:22, Null0

lets say this is router1

Notice that the summary route is sourced from Null0, and not an actual interface. That is because this route is used for advertisement purposes and does not represent a path that router1 can take to reach that network. On router1, this route has an administrative distance of 5

so any router connected to router one can send packect going to any address within the summary address range and router1 will have route for the more spesific route when come to it like in our example 2.1.1.0/24 through interface 1 and 2.1.2.0/24 through interface 2 and so on, but the nieghbor routers will see only the summary route and router1 as a next-hop

this is useful with larg routing tables

thank you

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[tdrais]

If all addresses have more specific routes on the summary router then the null0 is not really needed and will have no effect. It is used in case part of the network being summarized does not exist.

For example say I have

10.10.2.0/24 and 10.10.3.0/24 but I summarize it to 10.10.2.0/23. This will generate a 10.10.2.0/23 null0 route in the summary router but since I have routes to both the /24 these will override the null0 since they are more specific.

Now lets say I do the same summary but 10.10.3.0/24 is down and therefore not in the routing table. A packet sent to 10.10.3.x will now use the 10.10.2.0/23 null0 route and the packet will be dropped. This is mostly used to prevent routing loops especially when you have default routes configured.

[Marwan ALshawi]

hi tim

but the source of

"Notice that the summary route is sourced from Null0, and not an actual interface. That is because this route is used for advertisement purposes and does not represent a path that router1 can take to reach that network. On router1, this route has an administrative distance of 5 "

is cisco networking academy !

however ur idea is reasonable as well :)

[Danilo Dy]

You will be surprise the many use of Null0.

- Blackhole

- EIGRP route summarization

- BGP route aggregation

- NAT

- Routing loops

Those so far I found out and use over so many years in networking.