OSPF虚链路现象分析[文档]
本文档目的：在等级二的
实验手册P44上的逻辑图的基础上，
更深入理解虚链路的逻辑；弄清楚本实验中的R3在逻辑上到底是什么位置；看懂实验手册的逻辑图，看清楚实验手册上，那条虚线两端接口所属的区域。
实验设计：
配置设计：
R1#show run
Building configuration...
!
hostname R1
!
interface Loopback0
ip address 1.1.1.1 255.255.255.0
ip ospf network point-to-point
!
interface Serial1/0
ip address 12.1.1.1 255.255.255.0
!
router ospf 10
network 1.1.1.1 0.0.0.0 area 0
network 12.1.1.0 0.0.0.255 area 0
!
end
R2#show run
Building configuration...
!
hostname R2
!
interface Loopback0
ip address 2.2.2.2 255.255.255.0
ip ospf network point-to-point
!
interface Serial1/0
ip address 23.1.1.2 255.255.255.0
!
interface Serial1/1
ip address 12.1.1.2 255.255.255.0
!
router ospf 10
log-adjacency-changes
area 1 virtual-link 3.3.3.3
network 12.1.1.0 0.0.0.255 area 0
network 23.1.1.0 0.0.0.255 area 1
!
end
R3#show run
Building configuration...
Current configuration : 1081 bytes
!
hostname R3
!
interface Loopback0
ip address 3.3.3.3 255.255.255.0
ip ospf network point-to-point
!
interface Serial1/0
ip address 34.1.1.3 255.255.255.0
!
interface Serial1/1
ip address 23.1.1.3 255.255.255.0
!
router ospf 10
area 1 virtual-link 2.2.2.2
network 23.1.1.0 0.0.0.255 area 1
network 34.1.1.0 0.0.0.255 area 2
!
end
R4#show run
Building configuration...
Current configuration : 1039 bytes
!
hostname R4
!
interface Loopback0
ip address 4.4.4.4 255.255.255.0
ip ospf network point-to-point
!
interface Serial1/1
ip address 34.1.1.4 255.255.255.0
!
router ospf 10
log-adjacency-changes
network 4.4.4.4 0.0.0.0 area 2
network 34.1.1.0 0.0.0.255 area 2
!
end
实验现象：
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
34.0.0.0/24 is subnetted, 1 subnets
O IA 34.1.1.0 [110/192] via 12.1.1.2, 00:00:48, Serial1/0
1.0.0.0/24 is subnetted, 1 subnets
C 1.1.1.0 is directly connected, Loopback0
4.0.0.0/24 is subnetted, 1 subnets
O IA 4.4.4.0 [110/193] via 12.1.1.2, 00:00:48, Serial1/0
23.0.0.0/24 is subnetted, 1 subnets
O IA 23.1.1.0 [110/128] via 12.1.1.2, 00:00:48, Serial1/0
12.0.0.0/24 is subnetted, 1 subnets
C 12.1.1.0 is directly connected, Serial1/0
R2#show ip ro
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
O IA 34.1.1.0 [110/128] via 23.1.1.3, 00:01:54, Serial1/0
1.0.0.0/24 is subnetted, 1 subnets
O 1.1.1.0 [110/65] via 12.1.1.1, 00:01:54, Serial1/1
2.0.0.0/24 is subnetted, 1 subnets
C 2.2.2.0 is directly connected, Loopback0
4.0.0.0/24 is subnetted, 1 subnets
O IA 4.4.4.0 [110/129] via 23.1.1.3, 00:01:54, Serial1/0
23.0.0.0/24 is subnetted, 1 subnets
C 23.1.1.0 is directly connected, Serial1/0
12.0.0.0/24 is subnetted, 1 subnets
C 12.1.1.0 is directly connected, Serial1/1
R3#show ip ro
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, Serial1/0
1.0.0.0/24 is subnetted, 1 subnets
O 1.1.1.0 [110/129] via 23.1.1.2, 00:03:41, Serial1/1
3.0.0.0/24 is subnetted, 1 subnets
C 3.3.3.0 is directly connected, Loopback0
4.0.0.0/24 is subnetted, 1 subnets
O 4.4.4.0 [110/65] via 34.1.1.4, 00:04:31, Serial1/0
23.0.0.0/24 is subnetted, 1 subnets
C 23.1.1.0 is directly connected, Serial1/1
12.0.0.0/24 is subnetted, 1 subnets
O 12.1.1.0 [110/128] via 23.1.1.2, 00:03:41, Serial1/1
R4#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, Serial1/1
1.0.0.0/24 is subnetted, 1 subnets
O IA 1.1.1.0 [110/193] via 34.1.1.3, 01:03:18, Serial1/1
4.0.0.0/24 is subnetted, 1 subnets
C 4.4.4.0 is directly connected, Loopback0
23.0.0.0/24 is subnetted, 1 subnets
O IA 23.1.1.0 [110/128] via 34.1.1.3, 01:04:07, Serial1/1
12.0.0.0/24 is subnetted, 1 subnets
O IA 12.1.1.0 [110/192] via 34.1.1.3, 01:03:18, Serial1/1
***
请注意R3路由表的路由条目***全部是本区域路由
***
在看R2的路由条目***O IA 34.1.1.0 [110/128] via 23.1.1.3, 00:01:54, Serial1/0
O IA 4.4.4.0 [110/129] via 23.1.1.3, 00:01:54, Serial1/0
下面我们分析虚链路的逻辑
实验现象分析：
1、ospf若设计成标准的两层机构，虚链路的作用就是恢复逻辑上的两层结构
2、配置虚链路后，R3上的路有表中的路由都是O，没有O IA。
说明在逻辑上，R3有属于Aear0的接口。逻辑上认为，这台路由器是主干区域连接其他区域的ABR。所以R3所连接的非主干区域的路由器R4可学到主干区域的路由条目。
从路由表上看一切迹象证明逻辑上是这样的，
帖子来自汤姆一通，yangrubing-itaa QQ383506128
