OSPF N1路由莫名其妙的加1的原因
R1
interface Loopback0
ip address 1.1.1.1 255.255.255.0
!
interface FastEthernet1/0
ip address 12.1.1.1 255.255.255.0
duplex auto
speed auto
!
router rip
version 2
network 1.0.0.0
network 12.0.0.0
no auto-summary
!
R2
interface Loopback0
ip address 2.2.2.2 255.255.255.0
!
interface FastEthernet0/0
ip address 12.1.1.2 255.255.255.0
duplex auto
speed auto
!
interface FastEthernet1/0
ip address 23.1.1.2 255.255.255.0
duplex auto
speed auto
!
router ospf 10
router-id 2.2.2.2
log-adjacency-changes
area 1 nssa
redistribute rip metric-type 1 subnets
network 2.2.2.2 0.0.0.0 area 1
network 23.1.1.2 0.0.0.0 area 1
!
router rip
version 2
redistribute ospf 10 metric 1
network 12.0.0.0
no auto-summary
R3
interface Loopback0
ip address 3.3.3.3 255.255.255.0
!
interface FastEthernet0/0
ip address 23.1.1.3 255.255.255.0
duplex auto
speed auto
!
interface FastEthernet1/0
ip address 34.1.1.3 255.255.255.0
duplex auto
speed auto
!
router ospf 10
router-id 3.3.3.3
log-adjacency-changes
area 1 nssa
network 3.3.3.3 0.0.0.0 area 0
network 23.1.1.3 0.0.0.0 area 1
network 34.1.1.3 0.0.0.0 area 0
因其他路由器对该试验的讨论分析没有影响，所以配置信息未列出
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 not set
34.0.0.0/24 is subnetted, 1 subnets
C 34.1.1.0 is directly connected, FastEthernet1/0
1.0.0.0/24 is subnetted, 1 subnets
O N1 1.1.1.0 [110/22] via 23.1.1.2, 00:02:44, FastEthernet0/0
2.0.0.0/32 is subnetted, 1 subnets
O 2.2.2.2 [110/2] via 23.1.1.2, 00:03:16, FastEthernet0/0
5.0.0.0/32 is subnetted, 1 subnets
O IA 5.5.5.5 [110/3] via 34.1.1.4, 00:03:16, FastEthernet1/0
23.0.0.0/24 is subnetted, 1 subnets
C 23.1.1.0 is directly connected, FastEthernet0/0
12.0.0.0/24 is subnetted, 1 subnets
O N1 12.1.1.0 [110/22] via 23.1.1.2, 00:02:46, FastEthernet0/0
45.0.0.0/24 is subnetted, 1 subnets
O IA 45.1.1.0 [110/2] via 34.1.1.4, 00:03:19, FastEthernet1/0
R2(config)#int lo 0
R2(config-if)#ip ospf cost 10
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 not set
34.0.0.0/24 is subnetted, 1 subnets
C 34.1.1.0 is directly connected, FastEthernet1/0
1.0.0.0/24 is subnetted, 1 subnets
O N1 1.1.1.0 [110/31] via 23.1.1.2, 00:00:19, FastEthernet0/0
2.0.0.0/32 is subnetted, 1 subnets
O 2.2.2.2 [110/11] via 23.1.1.2, 00:00:19, FastEthernet0/0
5.0.0.0/32 is subnetted, 1 subnets
O IA 5.5.5.5 [110/3] via 34.1.1.4, 00:00:19, FastEthernet1/0
23.0.0.0/24 is subnetted, 1 subnets
C 23.1.1.0 is directly connected, FastEthernet0/0
12.0.0.0/24 is subnetted, 1 subnets
O N1 12.1.1.0 [110/31] via 23.1.1.2, 00:00:22, FastEthernet0/0
45.0.0.0/24 is subnetted, 1 subnets
O IA 45.1.1.0 [110/2] via 34.1.1.4, 00:00:22, FastEthernet1/0
Loopback接口的cost值改变后，N1路由的metric值也随之改变。
Loopback 0的IP地址和OSPF的router-id是相一致的，但是OSPF的router-id是手工指定的，路由器并不知道采用的那个loopback接口的IP地址。
所以猜想如果有其他loopback接口，是否会影响到N1路由的metric值
当然其他loopback接口也需要宣告到OSPF区域，不然的话怎么改OSPF的接口COST值
R2(config)#int lo 10
R2(config-if)#ip add 20.1.1.1 255.255.255.0
R2(config-if)#ip ospf cost 9
R2(config)#router ospf 10
R2(config-router)#net 20.1.1.1 0.0.0.0 a 1
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 not set
34.0.0.0/24 is subnetted, 1 subnets
C 34.1.1.0 is directly connected, FastEthernet1/0
1.0.0.0/24 is subnetted, 1 subnets
O N1 1.1.1.0 [110/30] via 23.1.1.2, 00:00:02, FastEthernet0/0
2.0.0.0/32 is subnetted, 1 subnets
O 2.2.2.2 [110/11] via 23.1.1.2, 00:00:02, FastEthernet0/0
20.0.0.0/32 is subnetted, 1 subnets
O 20.1.1.1 [110/10] via 23.1.1.2, 00:00:02, FastEthernet0/0
5.0.0.0/32 is subnetted, 1 subnets
O IA 5.5.5.5 [110/3] via 34.1.1.4, 00:00:02, FastEthernet1/0
23.0.0.0/24 is subnetted, 1 subnets
C 23.1.1.0 is directly connected, FastEthernet0/0
12.0.0.0/24 is subnetted, 1 subnets
O N1 12.1.1.0 [110/30] via 23.1.1.2, 00:00:05, FastEthernet0/0
45.0.0.0/24 is subnetted, 1 subnets
O IA 45.1.1.0 [110/2] via 34.1.1.4, 00:00:05, FastEthernet1/0
因此可以得出结论，N1路由的metric莫名其妙加1的原因决定于NSSA区域ASBR的拥有最大IP地址的loopback接口（该接口需要宣告到OSPF区域）的Cost值，默认情况下Loopback接口的COST值是1，所以N1路由的metric会加1，假如没有loopback接口宣告到OSPF区域,N1路由的metric不会加1的
R2(config)#router ospf 10
R2(config-router)#
R2(config-router)#no net 2.2.2.2 0.0.0.0 a 1
R2(config-router)#no net 20.1.1.1 0.0.0.0 a 1
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 not set
34.0.0.0/24 is subnetted, 1 subnets
C 34.1.1.0 is directly connected, FastEthernet1/0
1.0.0.0/24 is subnetted, 1 subnets
O N1 1.1.1.0 [110/21] via 23.1.1.2, 00:00:02, FastEthernet0/0
5.0.0.0/32 is subnetted, 1 subnets
O IA 5.5.5.5 [110/3] via 34.1.1.4, 00:00:02, FastEthernet1/0
23.0.0.0/24 is subnetted, 1 subnets
C 23.1.1.0 is directly connected, FastEthernet0/0
12.0.0.0/24 is subnetted, 1 subnets
O N1 12.1.1.0 [110/21] via 23.1.1.2, 00:00:02, FastEthernet0/0
45.0.0.0/24 is subnetted, 1 subnets
O IA 45.1.1.0 [110/2] via 34.1.1.4, 00:00:04, FastEthernet1/0