OSPF中的forward address主要在广播共享网络中起作用（如以下拓扑图），该字段的作用类似于"第三方下一跳"的概念。
以下例子可以更具体地理解OSPF LSA5中的forward address字段：
R1：：
interface FastEthernet0/0
ip address 123.1.1.1 255.255.255.0
!
router ospf 110
network 123.1.1.1 0.0.0.0 area 0
R2:
interface FastEthernet0/0
ip address 123.1.1.2 255.255.255.0
!
router ospf 110
redistribute rip subnets
network 123.1.1.2 0.0.0.0 area 0
!
router rip
version 2
network 123.0.0.0
no auto-summary
R3：
interface FastEthernet0/0
ip address 123.1.1.3 255.255.255.0
!
interface FastEthernet1/0
ip address 34.1.1.3 255.255.255.0
!
router rip
version 2
network 34.0.0.0
network 123.0.0.0
no auto-summary
查看OSPF的LSA5中forward address：
R1#sh ip os database external
OSPF Router with ID (123.1.1.1) (Process ID 110)
Type-5 AS External Link States
Routing Bit Set on this LSA
LS age: 375
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 34.1.1.0 (External Network Number )
Advertising Router: 123.1.1.2
LS Seq Number: 80000003
Checksum: 0x304A
Length: 36
Network Mask: /24
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 20
Forward Address: 123.1.1.3
External Route Tag: 0
R1#sh 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 E2 34.1.1.0 [110/20] via 123.1.1.3, 00:12:45, FastEthernet0/0
123.0.0.0/24 is subnetted, 1 subnets
C
123.1.1.0 is directly connected, FastEthernet0/0
当把R2的F0/0接口passive或更改其OSPF的网络类型后，发现forward address为0.0.0.0，路由表中34网段的下一跳变为123.1.1.2即R2F0/0接口的地址。
结论：
1.forward address的作用：
forward address是ASBR通告的type 5 LSA中的字段，它是告诉OSPF域内的路由器如何选择下一跳地址才能够更快捷地到达LSA5所通告的路由。
2.forward address为0.0.0.0
此时路由器想去往外部路由就把包发往ASBR。
3.forward address为非0.0.0.0
此时路由表中的forward address网段若有效，则去往外部路由就发给该forward address地址。
4.满足forward address为非0.0.0.0必须同时满足以下条件：
a.引入的这条外部路由，其对应的出接口（如本例R2的F0/0）宣告OSPF。
b.引入的这条外部路由，其对应的出接口不被passive-interface。
c.引入的这条外部路由，其对应的出接口的OSPF网络类型必须为broadcast。