ospf summarization rules :

• can be done only at ABR&ASBR
• it’s done under the OSPF process
• between areas we use area-range command
• for external routes we use summary-address command 

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summarization between areas ( area-range command )

• command : area area-id range ip-address mask [advertise | not-advertise] [cost cost]
• by default, the metric will be the lowest metric among the summarized routes
• the ABR will have all routes
• the summarized route will appear as NULL0 to prevent routing loops.
• This command will only work on the first advertisor ABR
  • this command work on type1 & type 2 lsa only.
  • because of that we have to specify a directly connected area when issuing the command.
  • other than that the IOS will accept the command, but it has no effect

this is what R1 see

R1#show ip route
     172.17.0.0/24 is subnetted, 1 subnets
O IA    172.17.0.0 [110/74] via 10.0.0.2, 00:00:17, FastEthernet0/0
     172.16.0.0/24 is subnetted, 1 subnets
O IA    172.16.0.0 [110/74] via 10.0.0.2, 00:00:17, FastEthernet0/0
     172.19.0.0/24 is subnetted, 1 subnets
O IA    172.19.0.0 [110/74] via 10.0.0.2, 00:00:17, FastEthernet0/0
     172.18.0.0/24 is subnetted, 1 subnets
O IA    172.18.0.0 [110/74] via 10.0.0.2, 00:00:17, FastEthernet0/0
     172.20.0.0/24 is subnetted, 1 subnets
O IA    172.20.0.0 [110/74] via 10.0.0.2, 00:00:19, FastEthernet0/0
     10.0.0.0/24 is subnetted, 1 subnets
C       10.0.0.0 is directly connected, FastEthernet0/0

now in the ABR we type the following .

R2(config-router)#area 1 range 172.16.0.0 255.248.0.0

and let’s take a look at R1 routing table

R1#show ip route
     10.0.0.0/24 is subnetted, 1 subnets
C       10.0.0.0 is directly connected, FastEthernet0/0
O IA 172.16.0.0/13 [110/74] via 10.0.0.2, 00:04:01, FastEthernet0/0

now R1 know that any route between 172.16.0.0 to 172.23.0.0 is advertised by R2 .

so what about 172.21.0.0 to 172.23.0.0
lets how the ABR will deal with requests to those networks

R2#show ip route
     172.17.0.0/24 is subnetted, 1 subnets
C       172.17.0.0 is directly connected, Serial0/1
     172.16.0.0/24 is subnetted, 1 subnets
C       172.16.0.0 is directly connected, Serial0/0
     172.19.0.0/24 is subnetted, 1 subnets
C       172.19.0.0 is directly connected, Serial0/3
     172.18.0.0/24 is subnetted, 1 subnets
C       172.18.0.0 is directly connected, Serial0/2
     172.20.0.0/24 is subnetted, 1 subnets
C       172.20.0.0 is directly connected, Serial0/4
     10.0.0.0/24 is subnetted, 1 subnets
C       10.0.0.0 is directly connected, FastEthernet0/0
O    172.16.0.0/13 is a summary, 00:12:01, Null0 <-------

let’s say that the ABR received a packet with a destination 172.22.0.0/24, of course it will not match any real route, and if that ABR had a default route he could send out that packet and that packet keep circling.

to avoid such situation the ABR will add the summary route with NULL0, which mean discard the packet.

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External route summary (summary-address command)

• command : summary-address ip-prefix/length [ not-advertise ] [ tag tag ]
• this command will be applied to the advertising ASBR.
• the summarized route will appear as NULL0 to prevent routing loops.

for now it’s very easy to convert R2 from ABR to ASBR.

R2(config-router)#no network 172.16.0.0 0.7.255.255 area 1
R2(config-router)#
*Mar  1 01:04:13.819: %OSPF-5-ADJCHG: Process 1, Nbr 172.20.0.7 on Serial0/4 from FULL to DOWN, Neighbor Down: Interface down or detached
*Mar  1 01:04:13.819: %OSPF-5-ADJCHG: Process 1, Nbr 172.19.0.6 on Serial0/3 from FULL to DOWN, Neighbor Down: Interface down or detached
*Mar  1 01:04:13.819: %OSPF-5-ADJCHG: Process 1, Nbr 172.18.0.5 on Serial0/2 from FULL to DOWN, Neighbor Down: Interface down or detached
*Mar  1 01:04:13.819: %OSPF-5-ADJCHG: Process 1, Nbr 172.17.0.4 on Serial0/1 from FULL to DOWN, Neighbor Down: Interface down or detached
*Mar  1 01:04:13.819: %OSPF-5-ADJCHG: Process 1, Nbr 172.16.0.3 on Serial0/0 from FULL to DOWN, Neighbor Down: Interface down or detached

lets remove it from the diagram and make and add a new photo so you don’t scroll up

now lets first redistribute those connected routes into OSPF .

R2(config-router)#redistribute connected subnets

now let’s see the view from R1
well, it’s the same like before but E2 instead of IA

R1#show ip route

     172.17.0.0/24 is subnetted, 1 subnets
O E2    172.17.0.0 [110/20] via 10.0.0.2, 00:00:51, FastEthernet0/0
     172.16.0.0/24 is subnetted, 1 subnets
O E2    172.16.0.0 [110/20] via 10.0.0.2, 00:00:51, FastEthernet0/0
     172.19.0.0/24 is subnetted, 1 subnets
O E2    172.19.0.0 [110/20] via 10.0.0.2, 00:00:51, FastEthernet0/0
     172.18.0.0/24 is subnetted, 1 subnets
O E2    172.18.0.0 [110/20] via 10.0.0.2, 00:00:51, FastEthernet0/0
     172.20.0.0/24 is subnetted, 1 subnets
O E2    172.20.0.0 [110/20] via 10.0.0.2, 00:00:53, FastEthernet0/0
     10.0.0.0/24 is subnetted, 1 subnets
C       10.0.0.0 is directly connected, FastEthernet0/0

so let’s apply summarization.

R2(config-router)#summary-address 172.16.0.0 255.248.0.0

let’s see R1

R1#show ip route
     10.0.0.0/24 is subnetted, 1 subnets
C       10.0.0.0 is directly connected, FastEthernet0/0
O E2 172.16.0.0/13 [110/20] via 10.0.0.2, 00:03:22, FastEthernet0/0

R2 will have the same NULL0 route but no need to display the output since it’s the same .