OSPF路由汇总

OSPF路由汇总

在不使用路由汇总的情况下，每个链路的LSA会发送到OSPF骨干一级其他区域中，这就造成了不必要的网络流量和路由开销。但假如使用路由汇总，只有汇总的路由会传播到骨干区域，避免了大量LSA占用带宽，CPU以及内存资源。即便链路出现了故障，这种变化也不会传播到骨干网络中，使得骨干网络更加稳定。

OSPF有两种汇总方式：

域间汇总：

域间汇总在ABR上进行，命令为在ospf进程下，area 1 range 192.168.20.0 25.255.252.0

域间汇总演示

以下面拓扑图为例



对R1进行配置：

R1>en
R1#conf t
R1(config)#int lo 0
R1(config-if)#ip add 1.1.1.1 255.255.255.255
R1(config-if)#no sh
R1(config-if)#int f0/0
R1(config-if)#ip add 12.1.1.1 255.255.255.0
R1(config-if)#no sh
R1(config-if)#exi

R1(config)#router os 100
R1(config-router)#router-id 1.1.1.1
R1(config-router)#net 1.1.1.1 0.0.0.0 a 0
R1(config-router)#net 12.1.1.0 0.0.0.255 a 0
R1(config-router)#exi

对R2进行配置：

R2>en
R2#conf t
R2(config)#int lo 0
R2(config-if)#ip add 2.2.2.2 255.255.255.255
R2(config-if)#no sh
R2(config-if)#int f0/0
R2(config-if)#ip add 12.1.1.2 255.255.255.0
R2(config-if)#no sh
R2(config-if)#exi
R2(config)#int f0/1
R2(config-if)#ip add 23.1.1.2 255.255.255.0
R2(config-if)#no sh
R2(config-if)#exi
R2(config)#int f1/0
R2(config-if)#ip add 24.1.1.2 255.255.255.0
R2(config-if)#no sh
R2(config-if)#exi

R2(config)#router os 100
R2(config-router)#router-id 2.2.2.2
R2(config-router)#net 2.2.2.2 0.0.0.0 a 0
R2(config-router)#net 12.1.1.0 0.0.0.255 a 0
R2(config-router)#net 23.1.1.0 0.0.0.255 a 1
R2(config-router)#net 24.1.1.0 0.0.0.255 a 2
R2(config-router)#exi

对R3进行配置：

R3>en
R3#conf t
R3(config)#int lo 0
R3(config-if)#ip add 3.3.3.3 255.255.255.255
R3(config-if)#no sh
R3(config-if)#int lo 1
R3(config-if)#ip add 192.168.20.1 255.255.255.0
R3(config-if)#no sh
R3(config-if)#exi
R3(config)#int lo 2
R3(config-if)#ip add 192.168.21.1 255.255.255.0
R3(config-if)#no sh
R3(config-if)#int lo 3
R3(config-if)#ip add 192.168.22.1 255.255.255.0
R3(config-if)#no sh
R3(config-if)#int lo 4
R3(config-if)#ip add 192.168.23.1 255.255.255.0
R3(config-if)#no sh
R3(config-if)#exi
R3(config)#int f0/1
R3(config-if)#ip add 23.1.1.3 255.255.255.0
R3(config-if)#no sh
R3(config-if)#exi

R3(config)#router os 100
R3(config-router)#rou
4000
ter-id 3.3.3.3
R3(config-router)#net 3.3.3.3 0.0.0.0 a 1
R3(config-router)#net 192.168.20.1 0.0.0.255 a 1
R3(config-router)#net 192.168.21.1 0.0.0.255 a 1
R3(config-router)#net 192.168.22.1 0.0.0.255 a 1
R3(config-router)#net 192.168.23.1 0.0.0.255 a 1
R3(config-router)#net 23.1.1.0 0.0.0.255 a 1
R3(config-router)#exi

对R4进行配置：

R4>en
R4#conf t
R4(config)#int lo 0
R4(config-if)#ip add 4.4.4.4 255.255.255.255
R4(config-if)#no sh
R4(config-if)#int lo 1
R4(config-if)#ip add 192.168.32.1 255.255.255.0
R4(config-if)#no sh
R4(config-if)#int lo 2
R4(config-if)#ip add 192.168.33.1 255.255.255.0
R4(config-if)#no sh
R4(config-if)#int lo 3
R4(config-if)#ip add 192.168.34.1 255.255.255.0
R4(config-if)#no sh
R4(config-if)#int lo 4
R4(config-if)#ip add 192.168.34.1 255.255.255.0
R4(config-if)#no sh
R4(config-if)#int lo 5
R4(config-if)#ip add 192.168.36.1 255.255.255.0
R4(config-if)#no sh
R4(config-if)#int lo 6
R4(config-if)#ip add 192.168.36.1 255.255.255.0
R4(config-if)#no sh
R4(config-if)#int lo 7
R4(config-if)#ip add 192.168.38.1 255.255.255.0
R4(config-if)#no sh
R4(config-if)#int lo 8
R4(config-if)#ip add 192.168.38.1 255.255.255.0
R4(config-if)#no sh
R4(config-if)#int f0/1
R4(config-if)#ip add 24.1.1.4 255.255.255.0
R4(config-if)#no sh
R4(config-if)#exi

R4(config)#router os 100
R4(config-router)#router-id 4.4.4.4
R4(config-router)#net 4.4.4.4 0.0.0.0 a 2
R4(config-router)#net 192.168.32.0 0.0.0.255 a 2
R4(config-router)#net 192.168.33.0 0.0.0.255 a 2
R4(config-router)#net 192.168.34.0 0.0.0.255 a 2
R4(config-router)#net 192.168.35.0 0.0.0.255 a 2
R4(config-router)#net 192.168.36.0 0.0.0.255 a 2
R4(config-router)#net 192.168.37.0 0.0.0.255 a 2
R4(config-router)#net 192.168.38.0 0.0.0.255 a 2
R4(config-router)#net 192.168.39.0 0.0.0.255 a 2
R4(config-router)#net 24.1.1.0 0.0.0.255 a 2
R4(config-router)#exi

配置成功后我们在R1上看一下路由表：

R1#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

1.0.0.0/32 is subnetted, 1 subnets
C       1.1.1.1 is directly connected, Loopback0
2.0.0.0/32 is subnetted, 1 subnets
O       2.2.2.2 [110/11] via 12.1.1.2, 00:15:49, FastEthernet0/0
3.0.0.0/32 is subnetted, 1 subnets
O IA    3.3.3.3 [110/21] via 12.1.1.2, 00:13:26, FastEthernet0/0
4.0.0.0/32 is subnetted, 1 subnets
O IA    4.4.4.4 [110/12] via 12.1.1.2, 00:11:49, FastEthernet0/0
23.0.0.0/24 is subnetted, 1 subnets
O IA    23.1.1.0 [110/20] via 12.1.1.2, 00:15:48, FastEthernet0/0
192.168.38.0/32 is subnetted, 1 subnets
O IA    192.168.38.1 [110/12] via 12.1.1.2, 00:11:51, FastEthernet0/0
192.168.21.0/32 is subnetted, 1 subnets
O IA    192.168.21.1 [110/21] via 12.1.1.2, 00:13:29, FastEthernet0/0
24.0.0.0/24 is subnetted, 1 subnets
O IA    24.1.1.0 [110/11] via 12.1.1.2, 00:15:42, FastEthernet0/0
192.168.39.0/32 is subnetted, 1 subnets
O IA    192.168.39.1 [110/12] via 12.1.1.2, 00:11:53, FastEthernet0/0
192.168.20.0/32 is subnetted, 1 subnets
O IA    192.168.20.1 [110/21] via 12.1.1.2, 00:13:30, FastEthernet0/0
192.168.23.0/32 is subnetted, 1 subnets
O IA    192.168.23.1 [110/21] via 12.1.1.2, 00:13:30, FastEthernet0/0
192.168.36.0/32 is subnetted, 1 subnets
O IA    192.168.36.1 [110/12] via 12.1.1.2, 00:11:55, FastEthernet0/0
192.168.22.0/32 is subnetted, 1 subnets
O IA    192.168.22.1 [110/21] via 12.1.1.2, 00:13:33, FastEthernet0/0
192.168.37.0/32 is subnetted, 1 subnets
O IA    192.168.37.1 [110/12] via 12.1.1.2, 00:11:56, FastEthernet0/0
192.168.34.0/32 is subnetted, 1 subnets
O IA    192.168.34.1 [110/12] via 12.1.1.2, 00:11:57, FastEthernet0/0
12.0.0.0/24 is subnetted, 1 subnets
C       12.1.1.0 is directly connected, FastEthernet0/0
192.168.35.0/32 is subnetted, 1 subnets
O IA    192.168.35.1 [110/12] via 12.1.1.2, 00:11:58, FastEthernet0/0
192.168.32.0/32 is subnetted, 1 subnets
O IA    192.168.32.1 [110/12] via 12.1.1.2, 00:11:59, FastEthernet0/0
192.168.33.0/32 is subnetted, 1 subnets
O IA    192.168.33.1 [110/12] via 12.1.1.2, 00:12:00, FastEthernet0/0

我们可以看见，由于域间路由的传播，R1上多达进20条路由，有R3宣告的4个，R4宣告的8个192.68网段的路由如果不进行汇总，会泛洪到全网，所以我们应该在Area 1和Area 2的ABR上也就是R2上进行汇总操作，具体操作如下：

R2(config)#router os 100

R2(config-router)#area 1 range 192.168.20.0 255.255.252.0

R2(config-router)#area 2 range 192.168.32.0 255.255.248.0

R2(config-router)#exi

接下来我们去R1查看一下路由表：

R1#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

1.0.0.0/32 is subnetted, 1 subnets
C       1.1.1.1 is directly connected, Loopback0
2.0.0.0/32 is subnetted, 1 subnets
O       2.2.2.2 [110/11] via 12.1.1.2, 00:20:48, FastEthernet0/0
3.0.0.0/32 is subnetted, 1 subnets
O IA    3.3.3.3 [110/21] via 12.1.1.2, 00:18:26, FastEthernet0/0
4.0.0.0/32 is subnetted, 1 subnets
O IA    4.4.4.4 [110/12] via 12.1.1.2, 00:16:49, FastEthernet0/0
23.0.0.0/24 is subnetted, 1 subnets
O IA    23.1.1.0 [110/20] via 12.1.1.2, 00:20:48, FastEthernet0/0
24.0.0.0/24 is subnetted, 1 subnets
O IA    24.1.1.0 [110/11] via 12.1.1.2, 00:20:40, FastEthernet0/0
12.0.0.0/24 is subnetted, 1 subnets
C       12.1.1.0 is directly connected, FastEthernet0/0
O IA 192.168.20.0/22 [110/21] via 12.1.1.2, 00:00:39, FastEthernet0/0
O IA 192.168.32.0/21 [110/12] via 12.1.1.2, 00:00:18, FastEthernet0/0

可以看见，Area 1和Area 2上的192.168网段已经完成了汇总。这样会节省很多资源，拓扑发生变化也不会影响Area 0的路由表。

域外汇总

域外汇总需在ASBR上进行，命令为summary-address 192.168.20.0 25.255.252.0

域外汇总演示

我们拓扑图不变，但将Area 2变为EIGRP



在R2上面更改配置：

R2(config)#router os 100
R2(config-router)#no net 24.1.1.0 0.0.0.255 a 2
R2(config-router)#exi

R2(config)#router ei 90
R2(config-router)#no au
R2(config-router)#net 24.1.1.0 0.0.0.255
R2(config-router)#exi

在R4上更改：

R4(config)#no router os 100

R4(config)#router ei 90
R4(config-router)#no au
R4(config-router)#net 24.1.1.0 0.0.0.255
R4(config-router)#net 4.4.4.4 0.0.0.0
R4(config-router)#net 192.168.32.0 0.0.0.255
R4(config-router)#net 192.168.33.0 0.0.0.255
R4(config-router)#net 192.168.34.0 0.0.0.255
R4(config-router)#net 192.168.35.0 0.0.0.255
R4(config-router)#net 192.168.36.0 0.0.0.255
R4(config-router)#net 192.168.37.0 0.0.0.255
R4(config-router)#net 192.168.38.0 0.0.0.255
R4(config-router)#net 192.168.39.0 0.0.0.255
R4(config-router)#exi

之后我们在R2上进行双向重分发

R2(config)#router os 100
R2(config-router)#redistribute eigrp 90 subnets
R2(config-router)#exi

R2(config)#router ei 90
R2(config-router)#redistribute ospf 100 metric 1000 1 255 1 1500
R2(config-router)#exi

之后再R1查看路由表：

R1#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

1.0.0.0/32 is subnetted, 1 subnets
C       1.1.1.1 is directly connected, Loopback0
2.0.0.0/32 is subnetted, 1 subnets
O       2.2.2.2 [110/11] via 12.1.1.2, 00:01:31, FastEthernet0/0
3.0.0.0/32 is subnetted, 1 subnets
O IA    3.3.3.3 [110/21] via 12.1.1.2, 00:01:31, FastEthernet0/0
4.0.0.0/32 is subnetted, 1 subnets
O E2    4.4.4.4 [110/20] via 12.1.1.2, 00:01:31, FastEthernet0/0
23.0.0.0/24 is subnetted, 1 subnets
O IA    23.1.1.0 [110/20] via 12.1.1.2, 00:01:31, FastEthernet0/0
O E2 192.168.38.0/24 [110/20] via 12.1.1.2, 00:01:32, FastEthernet0/0
24.0.0.0/24 is subnetted, 1 subnets
O E2    24.1.1.0 [110/20] via 12.1.1.2, 00:01:32, FastEthernet0/0
O E2 192.168.39.0/24 [110/20] via 12.1.1.2, 00:01:33, FastEthernet0/0
O E2 192.168.36.0/24 [110/20] via 12.1.1.2, 00:01:34, FastEthernet0/0
O E2 192.168.37.0/24 [110/20] via 12.1.1.2, 00:01:34, FastEthernet0/0
O E2 192.168.34.0/24 [110/20] via 12.1.1.2, 00:01:34, FastEthernet0/0
12.0.0.0/24 is subnetted, 1 subnets
C       12.1.1.0 is directly connected, FastEthernet0/0
O E2 192.168.35.0/24 [110/20] via 12.1.1.2, 00:01:35, FastEthernet0/0
O E2 192.168.32.0/24 [110/20] via 12.1.1.2, 00:01:35, FastEthernet0/0
O E2 192.168.33.0/24 [110/20] via 12.1.1.2, 00:01:35, FastEthernet0/0
O IA 192.168.20.0/22 [110/21] via 12.1.1.2, 00:01:35, FastEthernet0/0

可以看见O E2标记的为外部的EIGRP路由，并没有汇总，下面我们在R2上进行域外路由的汇总：

R2(config)#router os 100
R2(config-router)#summary-address 192.168.32.0 255.255.248.0
R2(config-router)#exi

然后我们在R1上查看路由表：

R1#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

1.0.0.0/32 is subnetted, 1 subnets
C       1.1.1.1 is directly connected, Loopback0
2.0.0.0/32 is subnetted, 1 subnets
O       2.2.2.2 [110/11] via 12.1.1.2, 00:04:44, FastEthernet0/0
3.0.0.0/32 is subnetted, 1 subnets
O IA    3.3.3.3 [110/21] via 12.1.1.2, 00:04:44, FastEthernet0/0
4.0.0.0/32 is subnetted, 1 subnets
O E2    4.4.4.4 [110/20] via 12.1.1.2, 00:04:44, FastEthernet0/0
23.0.0.0/24 is subnetted, 1 subnets
O IA    23.1.1.0 [110/20] via 12.1.1.2, 00:04:44, FastEthernet0/0
24.0.0.0/24 is subnetted, 1 subnets
O E2    24.1.1.0 [110/20] via 12.1.1.2, 00:04:46, FastEthernet0/0
12.0.0.0/24 is subnetted, 1 subnets
C       12.1.1.0 is directly connected, FastEthernet0/0
O IA 192.168.20.0/22 [110/21] via 12.1.1.2, 00:04:47, FastEthernet0/0
O E2 192.168.32.0/21 [110/20] via 12.1.1.2, 00:00:08, FastEthernet0/0

ok！已经完成了汇总！

下放默认路由的方法

假如我们有一条通往Internet的默认路由，但我们如何将它下放到ospf域内的所有路由器呢，以免我们一台一台的配置，接下来我们还是按照上一个拓扑图来演示一下：



我们假定R2去往R4的就是默认路由，那么我们先在R2上面进行配置：

R2(config)#ip route 0.0.0.0 0.0.0.0 f1/0
R2(config)#do show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route

Gateway of last resort is 0.0.0.0 to network 0.0.0.0

1.0.0.0/32 is subnetted, 1 subnets
O       1.1.1.1 [110/11] via 12.1.1.1, 00:08:40, FastEthernet0/0
2.0.0.0/32 is subnetted, 1 subnets
C       2.2.2.2 is directly connected, Loopback0
3.0.0.0/32 is subnetted, 1 subnets
O       3.3.3.3 [110/11] via 23.1.1.3, 00:08:40, FastEthernet0/1
4.0.0.0/32 is subnetted, 1 subnets
D       4.4.4.4 [90/156160] via 24.1.1.4, 00:14:42, FastEthernet1/0
23.0.0.0/24 is subnetted, 1 subnets
C       23.1.1.0 is directly connected, FastEthernet0/1
D    192.168.38.0/24 [90/156160] via 24.1.1.4, 00:14:09, FastEthernet1/0
192.168.21.0/32 is subnetted, 1 subnets
O       192.168.21.1 [110/11] via 23.1.1.3, 00:08:41, FastEthernet0/1
24.0.0.0/24 is subnetted, 1 subnets
C       24.1.1.0 is directly connected, FastEthernet1/0
D    192.168.39.0/24 [90/156160] via 24.1.1.4, 00:14:05, FastEthernet1/0
192.168.20.0/32 is subnetted, 1 subnets
O       192.168.20.1 [110/11] via 23.1.1.3, 00:08:44, FastEthernet0/1
192.168.23.0/32 is subnetted, 1 subnets
O       192.168.23.1 [110/11] via 23.1.1.3, 00:08:44, FastEthernet0/1
D    192.168.36.0/24 [90/156160] via 24.1.1.4, 00:14:20, FastEthernet1/0
192.168.22.0/32 is subnetted, 1 subnets
O       192.168.22.1 [110/11] via 23.1.1.3, 00:08:45, FastEthernet0/1
D    192.168.37.0/24 [90/156160] via 24.1.1.4, 00:14:17, FastEthernet1/0
D    192.168.34.0/24 [90/156160] via 24.1.1.4, 00:14:28, FastEthernet1/0
12.0.0.0/24 is subnetted, 1 subnets
C       12.1.1.0 is directly connected, FastEthernet0/0
D    192.168.35.0/24 [90/156160] via 24.1.1.4, 00:14:26, FastEthernet1/0
D    192.168.32.0/24 [90/156160] via 24.1.1.4, 00:14:36, FastEthernet1/0
D    192.168.33.0/24 [90/156160] via 24.1.1.4, 00:14:33, FastEthernet1/0
S*   0.0.0.0/0 is directly connected, FastEthernet1/0
O    192.168.20.0/22 is a summary, 00:08:47, Null0
O    192.168.32.0/21 is a summary, 00:04:08, Null0

我们看见表姐为*的就是默认路由，已经配置完毕，下面我们在Area 0和Area 1进行下放默认路由。

R2(config)#router os 100

R2(config-router)#default-information originate

接下来分别在R1和R3上查看路由表：

R1#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route

Gateway of last resort is 12.1.1.2 to network 0.0.0.0

1.0.0.0/32 is subnetted, 1 subnets
C       1.1.1.1 is directly connected, Loopback0
2.0.0.0/32 is subnetted, 1 subnets
O       2.2.2.2 [110/11] via 12.1.1.2, 00:10:14, FastEthernet0/0
3.0.0.0/32 is subnetted, 1 subnets
O IA    3.3.3.3 [110/21] via 12.1.1.2, 00:10:14, FastEthernet0/0
4.0.0.0/32 is subnetted, 1 subnets
O E2    4.4.4.4 [110/20] via 12.1.1.2, 00:10:14, FastEthernet0/0
23.0.0.0/24 is subnetted, 1 subnets
O IA    23.1.1.0 [110/20] via 12.1.1.2, 00:10:15, FastEthernet0/0
24.0.0.0/24 is subnetted, 1 subnets
O E2    24.1.1.0 [110/20] via 12.1.1.2, 00:10:15, FastEthernet0/0
12.0.0.0/24 is subnetted, 1 subnets
C       12.1.1.0 is directly connected, FastEthernet0/0
O*E2 0.0.0.0/0 [110/1] via 12.1.1.2, 00:00:11, FastEthernet0/0
O IA 192.168.20.0/22 [110/21] via 12.1.1.2, 00:10:17, FastEthernet0/0
O E2 192.168.32.0/21 [110/20] via 12.1.1.2, 00:05:38, FastEthernet0/0

R3#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route

Gateway of last resort is 23.1.1.2 to network 0.0.0.0

1.0.0.0/32 is subnetted, 1 subnets
O IA    1.1.1.1 [110/21] via 23.1.1.2, 00:11:46, FastEthernet0/1
2.0.0.0/32 is subnetted, 1 subnets
O IA    2.2.2.2 [110/11] via 23.1.1.2, 00:11:46, FastEthernet0/1
3.0.0.0/32 is subnetted, 1 subnets
C       3.3.3.3 is directly connected, Loopback0
4.0.0.0/32 is subnetted, 1 subnets
O E2    4.4.4.4 [110/20] via 23.1.1.2, 00:11:46, FastEthernet0/1
23.0.0.0/24 is subnetted, 1 subnets
C       23.1.1.0 is directly connected, FastEthernet0/1
C    192.168.21.0/24 is directly connected, Loopback2
24.0.0.0/24 is subnetted, 1 subnets
O E2    24.1.1.0 [110/20] via 23.1.1.2, 00:11:47, FastEthernet0/1
C    192.168.20.0/24 is directly connected, Loopback1
C    192.168.23.0/24 is directly connected, Loopback4
C    192.168.22.0/24 is directly connected, Loopback3
12.0.0.0/24 is subnetted, 1 subnets
O IA    12.1.1.0 [110/20] via 23.1.1.2, 00:11:50, FastEthernet0/1
O*E2 0.0.0.0/0 [110/1] via 23.1.1.2, 00:01:46, FastEthernet0/1
O E2 192.168.32.0/21 [110/20] via 23.1.1.2, 00:07:12, FastEthernet0/1

我们都看到了O*E2的这条默认路由标记，因为下方的路由属于静态的路由，自然属于OSPF域外路由，所以标记为O E2，单又是默认路由，所以就是O*E2。

那如果我们没有在R2上配置默认路由，但我们想让其他设备将通往R2的路由设置为默认路由该如何进行配置呢？

首先我们将R2还原：

R2(config)#no ip route 0.0.0.0 0.0.0.0 f1/0

R2(config)#router os 100

R2(config-router)#no default-information originate

之后我们再去R1看一下路由表：

R1#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

1.0.0.0/32 is subnetted, 1 subnets
C       1.1.1.1 is directly connected, Loopback0
2.0.0.0/32 is subnetted, 1 subnets
O       2.2.2.2 [110/11] via 12.1.1.2, 00:16:32, FastEthernet0/0
3.0.0.0/32 is subnetted, 1 subnets
O IA    3.3.3.3 [110/21] via 12.1.1.2, 00:16:32, FastEthernet0/0
4.0.0.0/32 is subnetted, 1 subnets
O E2    4.4.4.4 [110/20] via 12.1.1.2, 00:16:32, FastEthernet0/0
23.0.0.0/24 is subnetted, 1 subnets
O IA    23.1.1.0 [110/20] via 12.1.1.2, 00:16:32, FastEthernet0/0
24.0.0.0/24 is subnetted, 1 subnets
O E2    24.1.1.0 [110/20] via 12.1.1.2, 00:16:34, FastEthernet0/0
12.0.0.0/24 is subnetted, 1 subnets
C       12.1.1.0 is directly connected, FastEthernet0/0
O IA 192.168.20.0/22 [110/21] via 12.1.1.2, 00:16:34, FastEthernet0/0
O E2 192.168.32.0/21 [110/20] via 12.1.1.2, 00:11:54, FastEthernet0/0

默认路由消失，然后我们在OSPF进程下进行下放默认路由：

R2(config-router)#default-information originate always

然后再去R1查看路由表：

R1#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route

Gateway of last resort is 12.1.1.2 to network 0.0.0.0

1.0.0.0/32 is subnetted, 1 subnets
C       1.1.1.1 is directly connected, Loopback0
2.0.0.0/32 is subnetted, 1 subnets
O       2.2.2.2 [110/11] via 12.1.1.2, 00:18:07, FastEthernet0/0
3.0.0.0/32 is subnetted, 1 subnets
O IA    3.3.3.3 [110/21] via 12.1.1.2, 00:18:07, FastEthernet0/0
4.0.0.0/32 is subnetted, 1 subnets
O E2    4.4.4.4 [110/20] via 12.1.1.2, 00:18:07, FastEthernet0/0
23.0.0.0/24 is subnetted, 1 subnets
O IA    23.1.1.0 [110/20] via 12.1.1.2, 00:18:08, FastEthernet0/0
24.0.0.0/24 is subnetted, 1 subnets
O E2    24.1.1.0 [110/20] via 12.1.1.2, 00:18:08, FastEthernet0/0
12.0.0.0/24 is subnetted, 1 subnets
C       12.1.1.0 is directly connected, FastEthernet0/0
O*E2 0.0.0.0/0 [110/1] via 12.1.1.2, 00:00:33, FastEthernet0/0
O IA 192.168.20.0/22 [110/21] via 12.1.1.2, 00:18:10, FastEthernet0/0
O E2 192.168.32.0/21 [110/20] via 12.1.1.2, 00:13:32, FastEthernet0/0

综上，共有两种方式下放默认路由：

在本地有默认路由的情况下

R2(config-router)#default-information originate

本地没有默认路由的情况下，强行下放默认路由

R2(config-router)#default-information originate always

默认cost=1并且是一个由LSA-5下放的