介绍：

本实验将探讨如果在区域0和NSSA相连时，存在多个ABR会出现的问题：
1.
对转发地址的抉择。
2.
如何确定哪台路由器在Type7转Type5时被抑制。
拓扑如下：在R5上做重发布的操作。

基本配置：

Router 1

hostname r1 interface Loopback0 ip address 1.1.1.1 255.255.255.0 ! interface Ethernet0/0 ip address 14.1.1.1 255.255.255.0 ! ------------连接R4 interface Serial2/0 ip address 13.1.1.1 255.255.255.0 serial restart-delay 0 !--------------连接R3 router ospf 10 log-adjacency-changes network 13.1.1.0 0.0.0.255 area 0 network 14.1.1.0 0.0.0.255 area 0 !

Router 3

hostname r3 ! interface Loopback0 ip address 3.3.3.3 255.255.255.0 ! interface Ethernet0/0 no ip address shutdown ! interface Serial1/0 ip address 13.1.1.3 255.255.255.0 serial restart-delay 0 !---------连接R1 interface Serial2/0 ip address 35.1.1.3 255.255.255.0 serial restart-delay 0 !----------连接R5 router ospf 10 log-adjacency-changes area 1 nssa network 13.1.1.0 0.0.0.255 area 0 network 35.1.1.0 0.0.0.255 area 1

Router 4

hostname r4 ! interface Loopback0 ip address 4.4.4.4 255.255.255.0 ! interface Ethernet0/0 ip address 14.1.1.4 255.255.255.0 ! ----------连接R1 interface Serial1/0 ip address 45.1.1.4 255.255.255.0 serial restart-delay 0 !-----------连接R5 router ospf 10 log-adjacency-changes area 1 nssa network 14.1.1.0 0.0.0.255 area 0 network 45.1.1.0 0.0.0.255 area 1 !

Router 5

hostname r5 interface Loopback0 ip address 5.5.5.5 255.255.255.0 ! interface Serial1/0 ip address 35.1.1.5 255.255.255.0 serial restart-delay 0 !--------连接R3 interface Serial2/0 ip address 45.1.1.5 255.255.255.0 serial restart-delay 0 !--------连接R4 router ospf 10 log-adjacency-changes area 1 nssa redistribute connected subnets network 35.1.1.0 0.0.0.255 area 1 network 45.1.1.0 0.0.0.255 area 1 !

1.对转发地址的抉择 r1#show ip ospf database external OSPF Router with ID (1.1.1.1) (Process ID 10) Type-5 AS External Link States Routing Bit Set on this LSA LS age: 446 Options: (No TOS-capability, DC) LS Type: AS External Link Link State ID: 5.5.5.0 (External Network Number ) Advertising Router: 4.4.4.4 LS Seq Number: 80000001 Checksum: 0x9CBA Length: 36 Network Mask: /24 Metric Type: 2 (Larger than any link state path) TOS: 0 Metric: 20 Forward Address: 35.1.1.5 External Route Tag: 0

r3#show ip ospf database external OSPF Router with ID (3.3.3.3) (Process ID 10) Type-5 AS External Link States Routing Bit Set on this LSA LS age: 489 Options: (No TOS-capability, DC) LS Type: AS External Link Link State ID: 5.5.5.0 (External Network Number ) Advertising Router: 4.4.4.4 LS Seq Number: 80000001 Checksum: 0x9CBA Length: 36 Network Mask: /24 Metric Type: 2 (Larger than any link state path) TOS: 0 Metric: 20 Forward Address: 35.1.1.5 External Route Tag: 0

r4#show ip ospf database external OSPF Router with ID (4.4.4.4) (Process ID 10) Type-5 AS External Link States LS age: 277 Options: (No TOS-capability, DC) LS Type: AS External Link Link State ID: 5.5.5.0 (External Network Number ) Advertising Router: 4.4.4.4 LS Seq Number: 80000001 Checksum: 0x9CBA Length: 36 Network Mask: /24 Metric Type: 2 (Larger than any link state path) TOS: 0 Metric: 20 Forward Address: 35.1.1.5 External Route Tag: 0

为什么ADV是4.4.4.4而Forward Address却是35.1.1.5呢？它应该是45.1.1.5才对啊！我们初步（1）假定是假如一台路有器有多个接口，那么运行OSPF的最小接口的将被选为Forward Address，（此例中R5的两个接口是35.1.1.5和45.1.1.5，那么选择的是较小的地址35.1.1.5），现在来验证把R3、R5的接口地址改为53.1.1.0的，从而使他大于45.1.1.5的。但是结果却是还是选择53.1.1.5，从此可以推翻上述结论（不过这个实验我有做出那个地址小选那个的效果）

r1#show ip ospf database external OSPF Router with ID (1.1.1.1) (Process ID 10) Type-5 AS External Link States Routing Bit Set on this LSA LS age: 234 Options: (No TOS-capability, DC) LS Type: AS External Link Link State ID: 5.5.5.0 (External Network Number ) Advertising Router: 4.4.4.4 LS Seq Number: 80000001 Checksum: 0x63E1 Length: 36 Network Mask: /24 Metric Type: 2 (Larger than any link state path) TOS: 0 Metric: 20 Forward Address: 53.1.1.5 External Route Tag: 0

上述结论不成立的话，那我们进一步推测（2）可能是会不会跟R5和R3、R4形成邻居的先后关系有影响呢，是不是那个邻居先起来，那么对应的接口就优先呢，做个测试：
现在将R3、R5的邻居关系手动down ，将R5、R3的接口关闭，关闭以后Forward Address变为45.1.1.5

r1#show ip ospf database external OSPF Router with ID (1.1.1.1) (Process ID 10) Type-5 AS External Link States Routing Bit Set on this LSA LS age: 6 Options: (No TOS-capability, DC) LS Type: AS External Link Link State ID: 5.5.5.0 (External Network Number ) Advertising Router: 4.4.4.4 LS Seq Number: 80000002 Checksum: 0x943 Length: 36 Network Mask: /24 Metric Type: 2 (Larger than any link state path) TOS: 0 Metric: 20 Forward Address: 45.1.1.5 External Route Tag: 0

现在在将接口复原看效果，现在R5和R4先形成邻接，而R3、R5落后前者，但是Forward Address由45.1.1.5立即变为53.1.1.5，所以推论还是错误的。所以期待高人解答，现象极其没有规律，一次一个效果。（可能是个Bug也说不定）

r1#show ip ospf database external OSPF Router with ID (1.1.1.1) (Process ID 10) Type-5 AS External Link States Routing Bit Set on this LSA LS age: 15 Options: (No TOS-capability, DC) LS Type: AS External Link Link State ID: 5.5.5.0 (External Network Number ) Advertising Router: 4.4.4.4 LS Seq Number: 80000003 Checksum: 0x5FE3 Length: 36 Network Mask: /24 Metric Type: 2 (Larger than any link state path) TOS: 0 Metric: 20 Forward Address: 53.1.1.5 External Route Tag: 0

2.我们探讨第二个问题：

在R1上看关于重发布进来的5.5.5.0的数据库：

r1#show ip ospf database external OSPF Router with ID (1.1.1.1) (Process ID 10) Type-5 AS External Link States Routing Bit Set on this LSA LS age: 55 Options: (No TOS-capability, DC) LS Type: AS External Link Link State ID: 5.5.5.0 (External Network Number ) Advertising Router: 4.4.4.4 LS Seq Number: 80000001 Checksum: 0xB42 Length: 36 Network Mask: /24 Metric Type: 2 (Larger than any link state path) TOS: 0 Metric: 20 Forward Address: 45.1.1.5 External Route Tag: 0

在R3上看关于重发布进来的5.5.5.0的数据库：

r3# show ip ospf database external OSPF Router with ID (3.3.3.3) (Process ID 10) Type-5 AS External Link States Routing Bit Set on this LSA LS age: 85 Options: (No TOS-capability, DC) LS Type: AS External Link Link State ID: 5.5.5.0 (External Network Number ) Advertising Router: 4.4.4.4 LS Seq Number: 80000001 Checksum: 0xB42 Length: 36 Network Mask: /24 Metric Type: 2 (Larger than any link state path) TOS: 0 Metric: 20 Forward Address: 45.1.1.5 External Route Tag: 0

在R4上看关于重发布进来的5.5.5.0的数据库：

r4#show ip ospf database external OSPF Router with ID (4.4.4.4) (Process ID 10) Type-5 AS External Link States LS age: 440 Options: (No TOS-capability, DC) LS Type: AS External Link Link State ID: 5.5.5.0 (External Network Number ) Advertising Router: 4.4.4.4 LS Seq Number: 80000001 Checksum: 0xB42 Length: 36 Network Mask: /24 Metric Type: 2 (Larger than any link state path) TOS: 0 Metric: 20 Forward Address: 45.1.1.5 External Route Tag: 0

在R1、R4、R3看来通告外部5.5.5.0的Advertising Router:都是4.4.4.4，按道理来说因为R4、R3都是ABR路由器，它们两个都应该通告（LSA7转LSA5）5.5.5.0这条外部路有才对，为什么只有R4通告呢，初步猜测 如果存在多个冗余的ABR路由器时，它们之间会抉择出一台路由器来担任这个外部路有的通告者(此例为R4),而其它的ABR路由器被抑制不对这条外部路有进行通告，当通告路由器失效以后被抑制的路由器将担此大任成为一个继任者，下边我们来验证这个问题：现在是R4来担任这个角色，如果R4down后，那么应该R3来代替它的角色。推测结论为router-id大的为担此重任，而router-id小的将成为继任者。现在做个验证将R3的router-id改为6.6.6.6 〉4.4.4.4 ADV立刻被router-id高的路由器抢夺

r1# show ip ospf database external OSPF Router with ID (1.1.1.1) (Process ID 10) Type-5 AS External Link States Routing Bit Set on this LSA LS age: 15 Options: (No TOS-capability, DC) LS Type: AS External Link Link State ID: 5.5.5.0 (External Network Number ) Advertising Router: 6.6.6.6 LS Seq Number: 80000001 Checksum: 0x2716 Length: 36 Network Mask: /24 Metric Type: 2 (Larger than any link state path) TOS: 0 Metric: 20 Forward Address: 45.1.1.5 External Route Tag: 0： 结论：所以router-id高的路由器担任这个转发角色，而其它router-id低的ABR都将被抑制。

不错不错，挺好的