To understand why an ABR installs the summary route to null 0, let’s imagine a really simple network with just two Routers: A and B with the following properties:
-Router A is in Area 0
-Router B is an ABR between Areas 0 and 1
-Router B has a default route that goes to Router A
Now, let’s say that Area 1 has the following networks: 172.16.0.0/24, 172.16.1.0/24, and 172.16.2.0/24. You want to summarize Area 1 on Router B into Area 0. To do this, you would use an “area range” command with the summary address of 172.16.0.0/22. Note: this summary address would include all of the following /24 networks: 172.16.0.0/24, 172.16.1.0/24, 172.16.2.0/24, and 172.16.3.0/24.
Let’s imagine what would happen if Router B did NOT create the null route for that summary. Suppose Router A gets a packet destined for 172.16.3.50. What will happen?
- The summary address from Router B includes the address of 172.16.3.50, so router A will send the packet to router B.
- Router B receives the packet, but recall from the scenario, that it doesn’t actually have the 172.16.3.0/24 network in Area 1. Since it doesn’t have a specific route to that network, Router B will use its default route back to Router A. Router B sends the packet to Router A.
- Router A sends it to back to Router B because of the summary route.
This back and forth will continue until the TTL expires, and this is a routing loop.
So what does the summary route to null do for us on Router B? As you can imagine, it changes the behavior in step #2 above. When Router A sends the packet to Router B, Router B simply drops it rather than sending it back to A.