EIGRP route to null0

Hi,

Can you please go into detail on why EIGRP creates a route to null0 when we configure a summary?

Thank you,

Victor

Hi Victor,

It’s done to prevent the router that is advertising the summary from forwarding any packets that it doesn’t have a route for. For example, let’s say we have two routers, R1 and R2. On R1 we have three interfaces with these networks:

  • 192.168.0.0/24
  • 192.168.1.0/24
  • 192.168.3.0/24
Now on R1 we configure summary route 192.168.0.0/22 and advertise it to R2. When R2 has an IP packet for 192.168.2.x then it will forward it to R1.

What does R1 do with it? It doesn’t have an entry for 192.168.2.x in its routing table so it will drop the packet, no problems there.

Now let’s say that R1 is also connected to an ISP, it has a default route pointing to the ISP.

Once R1 receives a packet for 192.168.2.x, it will check its routing table and will use the default route, forwarding the packet for 192.168.2.x to the ISP. To prevent this from happening, EIGRP generates an entry that matches the summary route, pointing to null0. When you receive something for 192.168.2.x, R1 will drop the packet instead of using the default route.

Does this help?

Rene

1 Like

Yes this help a lot thank you!

If you make a summary route, there might be some addresses which don’t exist , even whole networks.

in that case, a device could try to ping an address in the summary propogation for some “unreal network” which doesnt apear for real.

the summary address is just advertised for those summarizedz networks but when the router receive a packet for those networks, he will have his own higher prefix lentgh routes in the RIB so as long he has those longer prefixes than the summary it self - he knows to route them.
if he finds out the longest prefix match is the actualy summary - it understand there isn’t a real summarized network for that ip addres so it should be discarded, otehrwise it will make a loop in the network if the summary point stoward some interface.