When you create summaries in EIGRP, the router automatically creates a route that points to a Null 0 interface.
This Null 0 interface serves as a bit bucket to prevent the router from forwarding traffic for destinations inside the summary address that it doesn’t have a longer match for. This is also known as Null routed.
This is true for OSPF and BGP summarization as well..
The administrative distance for summary routes have an AD of 5 by default. Knowing this we can configure a route with an AD of 1-4 to take precedence over our summary route.
Here is what I mean.
A simple EIGRP topology with R4, R5, and R8. Full EIGRP connectivity between nodes.
Note: Lo1 has been created on R4 and R5 and advertised into the EIGRP process.
Lets configure a default route on R4 Ethernet segment towards R5.
On R5, let’s check for the route to R4 Lo1 – 184.108.40.206. R5s CEF table matches our summary route to reach 220.127.116.11
Let’s check on R8. R8 just like R5 has a CEF entry for 18.104.22.168 that matches the summary route 0.0.0.0/0 that R4 generated. R8 and R5 don’t have a specific route for 22.214.171.124 because the summary route it suppressing the more specific route from being install into their routing tables, which is why we need to consult the CEF for the route.
R8 can pint 126.96.36.199 as of now.
Let’s configure a summary address for the Lo1 prefix’s on R5 for R8 on the shared Ethernet segment between them.
Below you can see that R8 received our new summary address for the Lo1 prefixes. We can see this in the out put 188.8.131.52/23. Lets try to ping 184.108.40.206 – R4s Lo1.
Pings to R4 – 220.127.116.11 fail. Pings to R5 – 18.104.22.168 are successful because R5 has this address locally configured.
Lets see what R5 has to say about the route to 22.214.171.124.
We can see that R5 can reach 126.96.36.199 via Null 0. Lets ping 188.8.131.52.
This can’t work! Our packets are Null routed!
So to get around this lets configure a static route on R5 to R4 for 184.108.40.206.
R5 now knows about 220.127.116.11 with a AD of 1. Let’s see if this fixes R8s reachability.
R8 can reach 18.104.22.168!
This could have also been solved by poisoning the Null 0 interface from R5s RIB with an AD of 255.
The difference here is that the Null 0 interface found on R5 is completely removed from the routing table.