You also have to know the "side effects" of route summarization. With OSPF, there will actually be an extra interface created at the point of summarization, and this catches a lot of CCNP candidates by surprise. Let's take a look at the null0 interface and how it relates to OSPF summarization.
On R1, the following networks are redistributed into OSPF, and then summarized.
The summary address appears on R2, a downstream router.
R2#show ip route ospf
O E2 16.0.0.0/6 [110/20] via 172.12.123.1, 00:00:05, Serial0
Let's go back to R1 and look at its OSPF table.
R1#show ip route ospf
O 16.0.0.0/6 is a summary, 00:01:51, Null0
Where did the null0 interface come from, and why is it there? Packets sent to the null interface are dropped, and in this case, that's a good thing.
When you configure summary routes in OSPF, a route to null0 will be installed into the OSPF routing table. This helps to prevent routing loops. Any packets destined for the routes that have been summarized will have a longer match in the routing table, as shown below...
C 17.0.0.0/8 is directly connected, Loopback17
C 16.0.0.0/8 is directly connected, Loopback16
C 19.0.0.0/8 is directly connected, Loopback19
C 18.0.0.0/8 is directly connected, Loopback18
O 16.0.0.0/6 is a summary, 00:01:51, Null0
.. and packets that do not match one of the summarized routes but do match the summary route will be dropped.
Preventing routing loops when performing route redistribution and summarization is vital. OSPF gives us a little help in that regard in this situation, and as you study more complex redistribution scenarios on your way to the CCNP and CCIE, you'll realize that we'll take all the help we can get!
