一个小实验，观察OSPF虚链路、虚链路的替代办法。
拓扑图：
如上图所示，右边 AREA 2、3、4 普通区域被 AREA 1 分割。
要求：不改动物理连线的方式配置设备，使得 R1 学习到 R6、R7、R8上的路由。
方式一：R2 – R3 间配置跨 AREA 1 的虚链路
方式二：R2 – R4 间配置 Tunnel 隧道，配置宣告
方式三：R5 上 配置 AREA 4 在另一个 OSPF 进程，配置重分布
方式一
!--- R2 commands.
router ospf 100
area 1 virtual-link 3.3.3.3
!--- R3 commands.
router ospf 100
area 1 virtual-link 2.2.2.2
方式二
!--- R2 commands.
!
interface Tunnel0
ip address 100.1.1.1 255.255.255.252
tunnel source 24.1.1.2
tunnel destination 24.1.1.4
!
router ospf 100
# 将Tunnel接口地址宣告进区域0
network 100.1.1.1 0.0.0.0 area 0
!
!--- R4 commands.
interface Tunnel0
ip address 100.1.1.2 255.255.255.252
tunnel source 24.1.1.4
tunnel destination 24.1.1.2
!
router ospf 100
# 将Tunnel接口地址宣告进区域0
network 100.1.1.2 0.0.0.0 area 0
!
方式三
R5#sh run | s r o
router ospf 100
router-id 5.5.5.5
# 重分布
redistribute ospf 200 subnets
network 5.5.5.5 0.0.0.0 area 1
network 25.1.1.5 0.0.0.0 area 1
# 配置 AREA 4 在另一个 OSPF 进程
router ospf 200
router-id 55.55.55.55
# 重分布
redistribute ospf 100 subnets
network 58.1.1.5 0.0.0.0 area 4
R5#
观察
R1 上查看 OSPF 路由，发现能学习到 R6，R7，R8上的环回口路由；Tunnel接口的地址也被学习到，而且是 O 表项路由属于区域 0 ，R8 的路由因为在R5 上进行了双向重分布显示为 OE2 表项。 R6，R7 则是区域间路由，逻辑上的直连区域0。
R1#sh ip route ospf
Codes: L - local, 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, H - NHRP, l - LISP
a - application route
+ - replicated route, % - next hop override, p - overrides from PfR
Gateway of last resort is not set
2.0.0.0/32 is subnetted, 1 subnets
O 2.2.2.2 [110/11] via 12.1.1.2, 00:26:14, Ethernet0/0
3.0.0.0/32 is subnetted, 1 subnets
O IA 3.3.3.3 [110/21] via 12.1.1.2, 00:25:54, Ethernet0/0
4.0.0.0/32 is subnetted, 1 subnets
O IA 4.4.4.4 [110/21] via 12.1.1.2, 00:26:03, Ethernet0/0
5.0.0.0/32 is subnetted, 1 subnets
O IA 5.5.5.5 [110/21] via 12.1.1.2, 00:26:03, Ethernet0/0
6.0.0.0/32 is subnetted, 1 subnets
O IA 6.6.6.6 [110/31] via 12.1.1.2, 00:25:54, Ethernet0/0
7.0.0.0/32 is subnetted, 1 subnets
O IA 7.7.7.7 [110/1021] via 12.1.1.2, 00:25:53, Ethernet0/0
8.0.0.0/32 is subnetted, 1 subnets
O E2 8.8.8.8 [110/11] via 12.1.1.2, 00:25:53, Ethernet0/0
23.0.0.0/24 is subnetted, 1 subnets
O IA 23.1.1.0 [110/20] via 12.1.1.2, 00:26:14, Ethernet0/0
24.0.0.0/24 is subnetted, 1 subnets
O IA 24.1.1.0 [110/20] via 12.1.1.2, 00:26:14, Ethernet0/0
25.0.0.0/24 is subnetted, 1 subnets
O IA 25.1.1.0 [110/20] via 12.1.1.2, 00:26:14, Ethernet0/0
36.0.0.0/24 is subnetted, 1 subnets
O IA 36.1.1.0 [110/30] via 12.1.1.2, 00:25:54, Ethernet0/0
47.0.0.0/24 is subnetted, 1 subnets
O IA 47.1.1.0 [110/1020] via 12.1.1.2, 00:26:14, Ethernet0/0
58.0.0.0/24 is subnetted, 1 subnets
O E2 58.1.1.0 [110/10] via 12.1.1.2, 00:25:54, Ethernet0/0
100.0.0.0/30 is subnetted, 1 subnets
O 100.1.1.0 [110/1010] via 12.1.1.2, 00:26:14, Ethernet0/0
R1#
R2上查看 OSPF 邻居，发现除常规的邻居外，还有Tunnel0 的邻居和虚链路的邻居，两种邻居的区别在于虚链路邻居一旦邻居建立处于稳定状态，不发送hello报文，而且Dead Time 永不超时。而Tunnel0 的邻居有Dead Time 默认40秒，debug 查看 hello 会发现*Mar 16 15:24:11.782: OSPF-100 HELLO Tu0: Rcv hello from 4.4.4.4 area 0 100.1.1.2，好像没什么用，好看
R2#sh ip os nei
Neighbor ID Pri State Dead Time Address Interface
4.4.4.4 0 FULL/ - 00:00:30 100.1.1.2 Tunnel0
1.1.1.1 1 FULL/BDR 00:00:32 12.1.1.1 Ethernet0/0
3.3.3.3 0 FULL/ - - 23.1.1.3 OSPF_VL0
5.5.5.5 1 FULL/DR 00:00:33 25.1.1.5 Ethernet0/3
4.4.4.4 1 FULL/DR 00:00:34 24.1.1.4 Ethernet0/2
3.3.3.3 1 FULL/DR 00:00:37 23.1.1.3 Ethernet0/1
R2#
R8上查看路由表，因为在R5上进行了重分布，R5是ASBR。所有看到的OSPF路由表项都是 OE
R8#sh ip rou os
Codes: L - local, 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, H - NHRP, l - LISP
a - application route
+ - replicated route, % - next hop override, p - overrides from PfR
Gateway of last resort is not set
1.0.0.0/32 is subnetted, 1 subnets
O E2 1.1.1.1 [110/21] via 58.1.1.5, 00:41:42, Ethernet0/0
2.0.0.0/32 is subnetted, 1 subnets
O E2 2.2.2.2 [110/11] via 58.1.1.5, 00:41:42, Ethernet0/0
3.0.0.0/32 is subnetted, 1 subnets
O E2 3.3.3.3 [110/21] via 58.1.1.5, 00:41:42, Ethernet0/0
4.0.0.0/32 is subnetted, 1 subnets
O E2 4.4.4.4 [110/21] via 58.1.1.5, 00:41:42, Ethernet0/0
5.0.0.0/32 is subnetted, 1 subnets
O E2 5.5.5.5 [110/1] via 58.1.1.5, 00:41:42, Ethernet0/0
6.0.0.0/32 is subnetted, 1 subnets
O E2 6.6.6.6 [110/31] via 58.1.1.5, 00:41:41, Ethernet0/0
7.0.0.0/32 is subnetted, 1 subnets
O E2 7.7.7.7 [110/31] via 58.1.1.5, 00:01:46, Ethernet0/0
12.0.0.0/24 is subnetted, 1 subnets
O E2 12.1.1.0 [110/20] via 58.1.1.5, 00:41:42, Ethernet0/0
23.0.0.0/24 is subnetted, 1 subnets
O E2 23.1.1.0 [110/20] via 58.1.1.5, 00:41:42, Ethernet0/0
24.0.0.0/24 is subnetted, 1 subnets
O E2 24.1.1.0 [110/20] via 58.1.1.5, 00:41:42, Ethernet0/0
25.0.0.0/24 is subnetted, 1 subnets
O E2 25.1.1.0 [110/10] via 58.1.1.5, 00:41:42, Ethernet0/0
36.0.0.0/24 is subnetted, 1 subnets
O E2 36.1.1.0 [110/30] via 58.1.1.5, 00:41:42, Ethernet0/0
47.0.0.0/24 is subnetted, 1 subnets
O E2 47.1.1.0 [110/30] via 58.1.1.5, 00:01:46, Ethernet0/0
100.0.0.0/30 is subnetted, 1 subnets
O E2 100.1.1.0 [110/1010] via 58.1.1.5, 00:41:42, Ethernet0/0
R8#
R8# sh ip os border-routers
OSPF Router with ID (8.8.8.8) (Process ID 200)
Base Topology (MTID 0)
Internal Router Routing Table
Codes: i - Intra-area route, I - Inter-area route
i 55.55.55.55 [10] via 58.1.1.5, Ethernet0/0, ASBR, Area 4, SPF 2
R8#