For route summarization to work, you need to know how to convert between binary and text, as well as how each protocol reacts to summarization. Chris Bryant, CCIE #12933, can tell you how OSPF does this.
To pass the CCNP exam, especially the BSCI exam, you need to know the details of route summarization. Not only do you need to be comfortable with binary conversions, but you also need to know how and where to use route summarization for each protocol.
You also need to know what happens when you sum up a route. At the point of summarization in OSPF, an extra interface is created, which catches a lot of CCNP candidates by surprise. Let's look at the null0 interface and how it relates to OSPF summarization.
On R1, the following networks are given new addresses in OSPF and then summed up.
Loopback16 is an interface.
Address: 16.16.16.16 255.0.0.0
loopback17 interface
loopback18 interface
Loopback19 is an interface.
Address: 19.19.19.19 255.0.0.0
#router R1(config) ospf 1
#Redistribute connected subnets on R1 (config-router)
Summary Address: 16.0.0.0 252.0.0.0 R1(config-router)#summary-address:
The summary address shows up on R2, a router that is further downstream.
show ip route ospf R2#
O E2 16.0.0.0/6 [110/20] via 172.12.123.1, 00:00:05, Serial0
Let's look at the OSPF table on R1 again.
R1#show ip route osspf
This is a summary of O 16.0.0.0/6, 00:01:51, Null0.
Why is the null0 interface there and where did it come from? When packets are sent to the null interface, they are dropped, which is a good thing in this case.
When summary routes are set up in OSPF, a route to null0 is added to the OSPF routing table. This helps to stop routing loops from happening. As shown below, any packets that are going to the routes that have been summed up will have a longer match in the routing table...
Loopback17 says that C 17.0.0.0/8 is directly connected.
C 16.0.0.0/8 is connected directly, Loopback16
C 19.0.0.0/8 is connected directly, Loopback19
C 18.0.0.0/8 is connected directly, Loopback18
This is a summary of O 16.0.0.0/6, 00:01:51, Null0.
... and packets that don't match any of the summarised routes but do match the summary route will be dropped.
When doing route redistribution and summarization, it is important to avoid routing loops. OSPF gives us a little help in this situation, and as you study more complex redistribution scenarios on your way to the CCNP and CCIE, you'll realise that we'll take all the help we can get!
