Null0 Interface when to use

itdaddy

hey guys.

can you explain to me when you would use or how can you tell you need a
say static route with the null0 interface (bit bucket; black hole for packets)???
how can you tell in the routing when you would need this and is it used lastly
in the routing table after all routes have been found???and before a default route????

networker050184

The router is going to treat the route to null like it would any other interface. If its the best route then it will use it.

When you need one depends on a lot of factors really.

xirtlook

I think it will cut down on traffic.

172.16.0.0/16 --> Null0
172.16.4.0/24 --> S0/0/0

anything destined for the 172.16.0.0 will be dropped unless a more specific path is destined (172.16.4..0). Rather than sending it out the default network or gateway of last resort. This frees up the network bandwidth.

I think thats the best way I can explain it. I'm sure there are more examples or someone who can explain it better than me. I'm still fairly new to the concept and use of the Null0 aswell, so hopefully someone else can chime in and explain it better.

Forsaken_GA

it's used by OSPF and EIGRP when you create a summary route to drop packets that don't have a better match than the summary, thereby ensuring theyre not continually routed until TTL expiration.

They're used to get routes in the table so they can be advertised, which is a requirement with BGP

They're used to purposely ensure that traffic destined to a prefix you dont like is dropped rather than routed

The uses for a bit bucket are many and varied

cisco_trooper

I too love routing unwanted destinations to null0.
I have used a static route to null0 simply so I could redistribute the static route into another routing protocol before, though I don't remember WHY I did that, I know I have done it.

I also sometimes use loopback interfaces at the far edges of my lab topologies to simulate hosts. I will source traffic from them and send them across the network, and I will traffic to them to verify end to end connectivity. This is a great lab use and it has come in quite handy for that purpose alone.

shodown

I had some where BGP issues where I had to have BGP peering going to one direction, but if the circuit went down I didnt' want it looping back around to peer from another member of the AS, so I used a null route on one side of the AS. Now that I'm reading what I did it seems so silly, but it was a good idea a the time,lol.

pham0329

shodown wrote: »

I had some where BGP issues where I had to have BGP peering going to one direction, but if the circuit went down I didnt' want it looping back around to peer from another member of the AS, so I used a null route on one side of the AS. Now that I'm reading what I did it seems so silly, but it was a good idea a the time,lol.

Not to go off-topic, but wouldn't that just be the default behavior of BGP? If your circuit goes down, the neighbor should also go down..unless you peer with a loopback?

If for whatever reason you needed to do NAT internally, I would think you want a Null 0 route so that it can be redistributed into your routing protocol.

shodown

well a lot of our sites had multiple circuits and we do peer to loopbacks, so the loopback would be reachable via another path when OSPF reconverged. for some reason which I can't remember now we didn't want the BGP to use that path which it would which is why we had null0 set so that the Peering would go live again. This was years ago when I thought I was a 50 pound brain.

gonzalito

I think this will help you ...
http://blogbt.net/index.php/2014/03/routing-to-null0-interface/