1.2.g Manual summarization with any routing protocol

EIGRP summarization 

Summarization can be employed on any router in an EIGRP network, since it is a distance-vector protocol and does not keep a LSDB on all routers on the network.  Summary routes in EIGRP have an AD value of 5.  The summary router will advertise the summary route with an outgoing interface of Null0 for loop-prevention purposes. 

EIGRP defaults to auto-summarisation on IOS versions older than 15, but versions after this do not auto-summary.  On older routers, disable auto summary under the EIGRP AS with command: no auto-summary. 

Configuration in classic mode applied to Interface

In this example we are summarizing four /24 networks into a /22

interface GigabitEthernet0/1
 ip summary-address eigrp 100 4.0.0.0/22

Configuration in named mode under the af-interface in the eigrp process

router eigrp MULTI-AF
 address-family ipv4 unicast autonomous-system 100
  af-interface GigabitEthernet0/0
   summary-address 4.0.0.0/22

OSPF summarization

As OSPF is a link-state protocol, summarization can only be performed on ABRs (Area Border Routers) and ASBRs (Autonomous System Border Routers). 

On an ABR we perform internal summarization of Type-1 LSAs into Type-3 LSA

Configuration for internal summarization:

router ospf 1
area <number> range <summary address> <subnet mask> [cost <cost value>]

The area number must be the area that has the network being specified, and the summary will then be advertised into all other areas by the ABR. 

On an ASBR we perform external summarization of Type-5 LSAs into Type-5 LSA and Type-7 LSAs into Type-7 LSA.

Configuration for external summarization:

router ospf 1
summary-address <summary address> <subnet mask>. 

BGP summarization

BGP is also capable of auto-summary and simply needs the command: auto-summary placed under the appropriate BGP AS (disabled by default).

auto-summary