EIGRP and Null interface

nezzy456

Hi,

I have just been reading about EIGRP.
It mentions that EIGRP places routes with null interfaces in its routing table, what is the benefit of this?

RIP also mentioned the use of null interfaces too but I cannot see why we would use them as they dont seem to do anything. I know there will be a reason but I cannot work it out.

Any info would be appreciated, thanks

tim100

A route pointing to the null0 interface is placed in the routing table whenever manual summarization is configured. For example, lets say you have these routes in your routing table:

192.168.4.0/24
192.168.5.0/24
192.168.6.0/24
192.168.7.0/24

Then you summarize these addresses to:

192.168.4.0/22

You will this route in your routing table pointing to Null0. Something like:

192.168.4.0/22 is a summary, 10:00:00, Null0

It's basically a discard route because it is a summary address.

kalebksp

Null interfaces are commonly used when summarizing multiple routes.

Say you have three network behind a router:
10.1.0.0/24
10.1.1.0/24
10.1.2.0/24

Then you create a summary route of 10.1.0.0/22. Since 10.1.0.0/22 encompasses the above routes and 10.1.3.0/24, any traffic destined to 10.1.3.0/24 will either be dropped or routed to the default route if one is installed. This would cause unnecessary traffic on the network, to prevent this situation when a summary route is created it is set to send traffic to the null interface. You probably know that traffic is always routed based on the most specific route so only traffic for which the router doesn't have a more specific route will be routed according to the 10.1.0.0/22 route pointing to null.

EDIT: Looks like tim100 beat me...

mikem2te

Someone correct me if I am wrong, I'm new to all this as well. What about routing loops. If there are four local interfaces on R1-

192.168.4.0/24
192.168.5.0/24
192.168.6.0/24
192.168.7.0/24
Summarizsed to:
192.168.4.0/22

This route will then get advertised to R2 which then forwards all traffic for these address to R1. All works so far.

Now if one of the interfaces goes down for example 192.168.6.0/24, R2 will forward traffic to R1 it gets sent to null / dropped. Still all good.

Suppose if the null interface is not there where will the traffic go for 192.168.6.0/24 if the interface is down. It would go to the default gateway if there is one defined. If this default gateway is to R2 - routing loop.

kryolla

mikem2te wrote: »

Suppose if the null interface is not there where will the traffic go for 192.168.6.0/24 if the interface is down. It would go to the default gateway if there is one defined. If this default gateway is to R2 - routing loop.

you got it

nezzy456

Thanks guys,

That really helps

mattau

sorry to dig up an old thread but saves me from recreating a new one.

I slowly am understanding these eigrp null0 things but in my labbing I have found something rather confusing

when I go in this exact order of commands the null either gets put in to the eigrp routing table or it doesnt for example

router eigrp 10
no auto-summary
network 10.10.10.1
etc etc etc

(the null routes get placed into the ip route table. BUT when i go

router eigrp 10
network 10.10.10.1
no autosummary

( no null routes are placed into the ip route table)

whats with this i didnt think it should matter ?

null routes are important arent they ? I will play on my live equipment tomorrow but i did this on gns3 and without null routes it crashed the program lol.

I purposely made 6 loopbacks summarized
192.168.0.1 to
192.168.5.1

summarized to 192.168.0.0 255.255.248.0

and tried to ping 192.168.6.1 as it would mean the route is saying 6.1 is in the summary but its really not and with no null im assuming it goes crazy and crashes everything because it doesnt know what to do with it ?

I have been googling all afternoon and found this post by someone on the cisco forums who says this

When you create the EIGRP summary you do not worry about null 0 because EIGRP will do that for us automatically.

If we create a manual static route then we must specify the next hop (in this discussion null 0). When we configure a summary-address in EIGRP then the null 0 is automatic.

does this sound right ? is it kind of like the implicit deny in ACL's? my gns3 is buggy at the best of times and crashes normally so I will test on live routers