LAB: OSPF Summarization and Discard Routes

- Configure R5 to advertise the summary 150.X.0.0/22 into area 3.
- Ensure that R5, SW2, and SW4 can still reach the Loopback0 networks of R1 and R2, even with the previously configured distribute-list filter applied.

when performing summarization in OSPF, similar with other protocols (EIGRP/BGP) a matching route to Null0 for the summary is installed locally in the routing table.  the "discard" route is used to prevent the forwarding traffic towards a shorter match, such as a default route if no specific route towards the actual destination.

command:

no discard-route [internal|external]

internal - refers to inter area summarization with area-range command
external - refers to redistributed summarization with summary-address command.

@ SW2 before adding the area-range command in R5

Rack1SW2#show ip route | include 150.
     150.1.0.0/16 is variably subnetted, 8 subnets, 2 masks
O IA    150.1.7.0/24 [110/97] via 155.1.58.5, 00:00:06, Vlan58
O IA    150.1.6.6/32 [110/96] via 155.1.58.5, 00:00:06, Vlan58
O       150.1.5.5/32 [110/31] via 155.1.58.5, 00:05:04, Vlan58
O IA    150.1.4.4/32 [110/95] via 155.1.58.5, 00:00:06, Vlan58
O IA    150.1.3.3/32 [110/95] via 155.1.58.5, 00:00:06, Vlan58
O       150.1.10.0/24 [110/151] via 155.1.108.10, 00:05:04, Port-channel1
O IA    150.1.9.0/24 [110/98] via 155.1.58.5, 00:00:06, Vlan58
C       150.1.8.0/24 is directly connected, Loopback0

Rack1R5(config)#router ospf 1
Rack1R5(config-router)#area 0 range 150.1.0.0 255.255.252.0

at SW2 after adding the area-range command in R5

Rack1SW2#show ip route | include 150.
     150.1.0.0/16 is variably subnetted, 8 subnets, 3 masks
O IA    150.1.7.0/24 [110/97] via 155.1.58.5, 00:02:00, Vlan58
O IA    150.1.6.6/32 [110/96] via 155.1.58.5, 00:02:00, Vlan58
O       150.1.5.5/32 [110/31] via 155.1.58.5, 00:06:58, Vlan58
O IA    150.1.4.4/32 [110/95] via 155.1.58.5, 00:02:00, Vlan58
O IA    150.1.0.0/22 [110/95] via 155.1.58.5, 00:00:06, Vlan58
O       150.1.10.0/24 [110/151] via 155.1.108.10, 00:06:58, Port-channel1
O IA    150.1.9.0/24 [110/98] via 155.1.58.5, 00:02:00, Vlan58
C       150.1.8.0/24 is directly connected, Loopback0


@ SW2 reachabilit to 150.1.3.3 is ok but not with 150.1.1.1


let us examine the R5's longest match to 150.1.1.1 is discard route via null0. which shows that the null0 is the longest match against the default route 0.0.0.0/0.

Rack1R5#show ip route 150.1.1.1
Routing entry for 150.1.0.0/22
  Known via "ospf 1", distance 110, metric 0, type intra area
  Routing Descriptor Blocks:
  * directly connected, via Null0
      Route metric is 0, traffic share count is 1

Rack1R5#show ip route 0.0.0.0
Routing entry for 0.0.0.0/0, supernet
  Known via "ospf 1", distance 110, metric 104, candidate default path
  Tag 1, type extern 1
  Last update from 155.1.0.4 on Serial0/0, 00:00:53 ago
  Routing Descriptor Blocks:
  * 155.1.45.4, from 150.1.4.4, 00:00:53 ago, via Serial0/1
      Route metric is 104, traffic share count is 1
      Route tag 1
    155.1.0.4, from 150.1.4.4, 00:00:53 ago, via Serial0/0
      Route metric is 104, traffic share count is 1
      Route tag 1

with disabling the discard route at R5.

Rack1R5(config)#router ospf 1
Rack1R5(config-router)#no discard-route internal

Rack1R5#show ip route 150.1.1.1
% Subnet not in table

Rack1R5#show ip route 0.0.0.0
Routing entry for 0.0.0.0/0, supernet
  Known via "ospf 1", distance 110, metric 104, candidate default path
  Tag 1, type extern 1
  Last update from 155.1.0.4 on Serial0/0, 00:00:16 ago
  Routing Descriptor Blocks:
  * 155.1.45.4, from 150.1.4.4, 00:00:16 ago, via Serial0/1
      Route metric is 104, traffic share count is 1
      Route tag 1
    155.1.0.4, from 150.1.4.4, 00:00:16 ago, via Serial0/0
      Route metric is 104, traffic share count is 1
      Route tag 1

final result is SW2 will use the 150.1.0.0/22 route as a result of the area-range command in R5 to reach 150.1.1.1 and R5 will use the 0.0.0.0/0 prefix to route the traffic towards R4.