How to perform route summarization using BGP protocol?

Route summarization is the process of replacing a series of routes into a summary route with subnet mask. The main purpose of summarization is to decrease the uses of bandwidth and make the size of routing table smaller. For example if you are trying to advertise four routes 194.1.17.0/24, 194.1.18.0/24, 194.1.19.0/24 and 194.1.20.0/24 from router C to router B then the only summarized address as 194.1.16.0 255.255.248.0 will be advertised in the update packege.
This tip will help you configure BGP route summarization between two different types of routing protocols. In this case, we will try to configure three different routers with routing protocols OSPF & RIP and then advertised one summary route from router C to router B using BGP Aggregate-address command.
Follow the network topology of three different class network in given figure:
Configuration to Enable OSPF & BGP on Router B
1)In global configuration Mode:
-B (config) # router ospf 1
-(Enable ospf protocol and 1 is Autonomous number)
-B (config-router) # network 40.0.0.0 0.255.255.255 area 0
-(Advertised router B Serial Network 40.0.0.0)
-B (config-router) # network 210.100.20.0 0.0.0.255 area 0
-(Advertised router B Ethernet Network 210.100.20.0)
-B (config-router) #redistribute bgp 20 subnet metric 50
-(Redistribute bgp into ospf)
-Now configure BGP on router B
-B(config)# router bgp 20
-B (config-router) # neighbor 61.1.1.2 remote-as 10
-B (config-router) #end
-Now run show command to display the BGP updates.
-B #show ip bgp
-Now verify the summarized route in router B routing table, run the command
-B #show ip route
-Configuration to Enable RIP & BGP on Router C
2)In global configuration Mode:
-C (config) # router rip
-(Enable rip protocol)
-C (config-router) # network 210.100.30.0
-(Advertised router C Serial Network 210.100.30.0)
-C(config)#ip route 40.0.0.0 255.0.0.0 61.1.1.1
-(Create a static route to router A to complete routing table.)
-Now create four static routes and then advertised in BGP.
-C(config)# ip route 194.1.17.0 255.255.255.0 null 0
-C(config)# ip route 194.1.18.0 255.255.255.0 null 0
-C(config)# ip route 194.1.19.0 255.255.255.0 null 0
-C(config)# ip route 194.1.20.0 255.255.255.0 null 0
-Now configure BGP on router C
-C(config)# router bgp 10
-C (config-router) # neighbor 61.1.1.1 remote-as 20
-C (config-router) # redistribute static
-(Advertised static routes)
-C (config-router) # aggregate-address 194.1.16.0 255.255.248.0 summary-only
-(summary of four static routes)
-C (config-router) #end
-Now run show command to display the BGP updates.
-C #show ip bgp
-Now verify the routes in router C routing table, run the command
-C #show ip route
-Configuration to Enable OSPF & BGP on Router A
3)In global configuration Mode:
-A (config) # router ospf 1
-(Enable ospf protocol and 1 is Autonomous number)
-A(config-router) # network 40.0.0.0 0.255.255.255 area 0
-(Advertised router A Serial Network 40.0.0.0)
-A(config-router) # network 210.100.10.0 0.0.0.255 area 0
-(Advertised router A Ethernet Network 210.100.10.0)