Summarization In EIGRP And RIP

Summarization in RIP and EIGRP can be configured manually using the IP summary-address command or automatically using auto-summary command. When auto summarization behaves in effect routers as shown in the steps below:

Subnets are summarized to their classful boundary before being sent to neighbors if they are not part of the major subnet of the exit interface. EIGRP automatically installs a discard route in the local routing table. RIP does not install a discard route by default.
In the following example we will explore EIGRP with automatic summarization enabled:

Router1 is connected to Router2 by a serial link and each router has loopback interfaces configured as shown in following diagram

Summarization
Router1 Configuration (same configuration applies to Router2)

R1(config)#router eigrp 10
R1(config-router)#auto-summary
R1(config-router)#network 131.108.0.0
R1(config-router)#network 137.99.0.0
Router1 routing table:

R1(config-router)#do sh ip route eigrp
137.99.0.0/16 is variably subnetted, 2 subnets, 2 masks
D 137.99.0.0/16 is a summary, 00:04:54, Null0 <– discard route
131.108.0.0/16 is variably subnetted, 4 subnets, 2 masks
D 131.108.3.0/24 [90/2297856] via 131.108.2.2, 00:06:00, Serial1/0 <– was not summarized
D 131.108.0.0/16 is a summary, 00:05:15, Null0 <– discard route
Router2 routing table:

R2(config-router)#do sh ip route eigrp
D 137.99.0.0/16 [90/2297856] via 131.108.2.1, 00:05:29, Serial1/0 <– Summarized subnet
131.108.0.0/24 is subnetted, 3 subnets
D 131.108.5.0 [90/2297856] via 131.108.2.1, 00:05:50, Serial1/0 <– non summarized route
Automatic summarization summarized:

R1 installs discard routes pointing to the null interface before sending summaries.
R1 sends the summarized route 137.99.0.0/16 to R2.
R1 sends 131.108.5.0/24 un-summarized because its part of the major subnet of the its exit interface.